I had so much fun with the Bath Alert project that I wrote an addon module so it can also be a shower timer! The laptop senses when you've turned on or off the water and then plays a Youtube of your choice when you've overstayed your intended time in the shower! This is an addon to the below instructable:
https://www.instructables.com/id/Alert-When-Your-Bath-is-Drawn/

You will need(pretty much all of the materials in the above instructable plus a couple more alligator clips and my second program):

A makey makey
Four 2 foot long alligator clips(should be included with the makey makey kit)
A bath tub
Less than 2 feet of duct tape
A zip lock bag(or you can destroy the anti-static bag in the kit)
A laptop
The Internet
Good Speakers(optional)

Submitted by Montana Ethical Hackers for the Instructables Sponsorship Program!

Step 1: Align the Alligator Clips, Install the Splash Shield, Wire the Makey Makey

It's unethical to repeat my material. Check out my instructable here. follow steps 1-3 and then come back here.

Ready? Okay! Let's make this into a shower timer as well!

Step 2: Add Two More Alligator Clips.

Clip an alligator clip to each electrode of the bath project. Tape the wires down to the far back floor of the shower. Keep them out of the way so you don't trip. These wires must have water between them when you're running the shower.

Download my Shower Timer program from this page. Unzip it and open it in a browser. Set the time you want your shower to last. Keep the window active and in the foreground while you take your shower.

You can reset the timer with the "Reset" button and you can test the timer with the space bar!

Happy showering! I'm going to take a shower and go to bed.

UPDATE: I fixed the timer so it's easier for people to use. It counts down in big minutes and seconds now and it works in all browsers... even in older versions of IE.