Here it is! My Electro Unicorn! Why? Because it might be the best idea I ever had. I was thinking of it for some time and first did it on a small scale here: https://www.instructables.com/id/DIY-RC-Unicorn/
This project was nice but it was just not enough. I wanted to ride one. So, the idea just escalated to a new level.

Following all the steps you should get a good overview of the project. There are for sure some details that are hard to explain, so watch both videos to get some extra ideas.

And of course, If you think I messed something feel free to comment and ask anything.

Step 1: Do You Want One? Sure, I Will Show You Hou to Make It!

It is going so well I could just wish for somebody else to have one, so we could ride them together.

So let's start.
First, you need an inflatable unicorn.
Nope, not the one with the covered center. I was looking for a solution how to mount a motor on that one and it was just not easy at all. Mostly because of the tail. And the weight of the motor not being in the center. I solved this problem by choosing a different one.

So you need a RING Unicorn. Finding that one saved me some engineering nightmares. The motor can be closer to the center, so the weight of the motor was not such a problem anymore. And it is way easier to fix since you cannot fix it in the front or back because of the unicorn's head/tail.

Mine was bought on ebay, but I am sure that's not an only option to get one. Depending on where you live you might also get it in store. But i have to say the "ring" ones are kind of difficult to find in such a big version. Most of them are too small. So yes, it needs to be as big as possible, since it is gonna be heavy.

Step 2: Choosing the Motor

Electric trolling motor is the best price/performance boat motor for such a project if you ask me.

There are other options, but I am not going to get into details, since I think they are not worth mentioning.

There are several different brands of trolling motors. Don't know the difference, so i just picked one, since most of them looked the same, just rebranded.

Trolling motors come in different power and voltage configurations.

From 36Lbs to 86Lbs of power in 12V or 24V DC versions.

I spent quite some time thinking how much power do I need? Choosing one of the smaller ones is just not the thing for me, so I was looking somewhere in the middle around 55Lbs on 12Volts seemed like a good compromise at first. But thinking a bit more the price difference from 55 to the biggest 86Lbs on 24V rated as 1200W was so little, that I just couldn't resist, so I picked that one.

When I got it I was shocked. It was big and heavy, and it just seemed impossible to fit it on the unicorn. But after some adjustments now looking back I think that was the perfect decision. There is never enough power :D

Step 3: The Platform

Because of the ring, I have to say this unicorn seems useless to me as a normal floatie. You cannot even sit normally on it. But it is perfect to get motorized. It just needs a platform.

I built it form 18mm spruce wood plates. 4 pieces 20cm wide and shaped to fit.

Take a look at the photos and you will get some ideas.

I did some reinforcements with fiberglass and wooden washers where the motor is fixed, drilled holes for the battery mount, screwed, glued together and painted in white.
Nothing special at all. It is the easyest solution for fixing the motor that came to my mind and if works just perfect.

Step 4: Motor Adjustments

Since the motor itself is big, I just started putting it apart to see how many parts I can get rid of to get it smaller and lighter.
I managed to got rid of the big handle, that contained only a speed switch. Also the main stick connecting the motor and handle was too long, so I cut away one part and used it as a steering handle later. (watch the video for details) I 3D printed the T part of the handle. There is a stl. file included if you want to copy my solution.
I replaced the speed switch with a DC motor driver from ebay. This way you can set the speed at any value, not just some 5prefixed ones. Also, the original switch drives the power threw some resistor coils to make the motor turn slower if you don't use full speed and this is a waste of power. There is less power loss with the dc PWM diver. I also unsoldered the original terminal blocks. They were poor quality. Not ready to handle 50A if you ask me. So, I just connected thick wires (red and black) instead and insulated the circuit board with liquid rubber.
The other 2 wires (yellow and white) are not in use for direct drive, so I insulated them.
I installed the power potentiometer on the handle as you can see from the photos. I will also add the stl. file for that if you want to print it.

Step 5: The Battery

This motor is supposed to run with 2x 12v Pb batteries. Or 1x 12V if you choose a lower power motor.
This will of course work, but is it not the best option for this project because lead batteries are heavy and big. No place for them here, except if you use smaller capacity ones for ups systems of motorbikes. This should work just fine. Connect them in series if you want 24V. If you don't know much about batteries this is still the best option for you.
If you are an advanced user of batteries you can go for LiPo as I did. I used 7S LiPo with 40Ah capacity. You can go for a pack of 18650 cells, pouch cells, anything that can handle 50A of current will do the job. I will not explain my battery box in detail, since if you are a pro battery user you know how to make a pack. If you are not I suggest you ask somebody who is for some friendly help, since lithium batteries at such volume are dangerous. They can easily catch fire if not handled properly.
Any batteries you choose, make sure to make them as waterproof as possible!
I made a glass fiber box for my battery and installed it under the platform using velcro and straps. I sealed with silicone all the cable glands, balance connector and the edge where the box is closed together.
Being mounted under the platform the battery box does get wet, but if sealed properly this shouldn't be a problem.
The DB 15 connector is used for balancing during charging and for cell status checks during the ride using a small display.

I am using this for sweet waters only. Sea water will damage the electronics much faster, so I am avoiding the sea for now. Maybe I will change my mind and go to the sea with it one day.
If you do go to the sea make sure you wash all the components when done riding.

Step 6: Steering

The unicorn's bottom part is O shaped, so unidirectional. This makes it hard to turn in water. For this I made a special "rudder" form 2mm fiberglass composite.
I did some tests mounting it in front and in the back. Go for the rear position! The photos show it on the front position, since they are form the building and testing period, but the front position makes you turn in the opposite direction as your motor steering handle.
Being mounted under the tail with some velcro straps it works perfect!

Step 7: Safety Wristband

Don't forget the safety. If you fall of you unicorn it will not stop! It may escape, or it may even hurt you if it runs over you with the motor.

My solution for this is the safety wristband. I split the on/off cable form the potentiometer and added a connector in between. The potentiometer has 2 connectors. You should split the 2 wire one and add an extra safety contact in series. This one is for the switch. The 3 wire one is for speed adjustment. The connector on the rope to the wristband is just a jumper. JST red 2 pin connectors are ok for that since they unplug easily, but you can use other connectors too.
So unless the jumper is plugged in, the motor doesn't run. As soon as you fall of you deatach it and the motor immediately stops.

Step 8: Final Assembly

I used some self-adhesive 3mm foam and covered the bottom part of the platform with it. Just to make double sure the wood is not too sharp when pressed to the floatie.
I installed 2 M6 screws to the motor mount plastic and drilled 2 holes in the platform, so the motor doesn't move from the platform at full power, since it is not made for front mount. It produces enough power to be easily pulled out of the platform without those screws.
The platform with the motor and the battery is heavy enough not to go anywhere, but just to be sure I used some straps to fix it to the unicorn's ring. First time I used 4 (2in front, 2 in the back), later only 2 on the side and they are more than enough.

That's it! Let's ride!
It's perfect if you ride it alone. Not enough space for 2 people on it. But it can easily tow as many friends on other floaties as you want!