I have my Garmin GPSMAP 62s attached to the deck of my Sea kayak right in front of the cockpit in a position that it is easily viewed and operated while paddling.

Despite the GPS being sold as IPX-7 (immersion to 1m for 30 minutes and splash protected) i had found on a couple of occasions that it started doing weird stuff after a particularly stormy day of splash, and a little water could be seen inside the USB cover.

So i figured i needed to devise a splash cover that fitted the following requirements;

Able to be used on the Marine mount
Screen fogging proof:-)
Easy to remove for land based operations
Easy to drain once the kayak is righted after a spill
Protect the unit from all splash

Step 1: Splash Cover for Kayak Deck Mount

What you need;

Take your device and mount to the store and find a clear dry bag that fits easily over your device on its mount( I used an Aqua Quest Watertight VHF bag)
Duct tape
Length of 4 mm Bungee cord
Bungee plastic hook
Cable tie & Whipping waxed thread
Naked flame (pen torch or candle)
Sissor's

Step 2: Modify the Dry Bag

The dry bag is designed to roll shut at the base to seal, but this function is not required in this mod as its only a splash guard and the free movement of air keeps the clear window condensation free.
Place your device on its mount, and slide the back over both until you hit the deck, pull it down enough so that all the operating buttons are in the clear window and there is a little bit of excess between the bottom of the device and the deck.
Mark the position where the "bottom of the skirt" touches the deck,(image a) and then cut off the remainder little at a time until you have it right.(image b)
I also had to cut a inch wide notch in the rear of the bag to accommodate the shame of the mount.

Step 3:

Cut a couple of lengths of duct tape and reinforce the bottom edges, including any back cuts needed.(image b)

The bungee

In my case my mount comes in to parts, the permanent part mounted to the kayak (see my Instructable here) and the part that connects to the device. I use the bungee for two purposes;

a) to keep the modified "splash bag" in position

b) as a safety lanyard connecting the device and the detachable section of the mount to the kayak.

So take one end of the bungee and using the naked flame, set light to one end.
Spit on your hands and blow out the flame once the tip is molten
Quickly mold the molten bit until the end is nicely sealed against fraying
Then i anchor that end of the bungee around the structure of the detachable part, and close it with a cable tie and a waxed line whipping.(image c)
I position the detachable section on its mount, and measure enough bungee cord to reach an attachment point on the Kayak close to the mount.
Put the hook on the bungee and melt the end to seal ( you may need to do this before depending on your hook type)
With the device on the mount, place the "splash guard" over and pull down to the position, then take the bungee and pass wrap around the base of the "skirt" a couple of times,(image d-1.) through a d buckle on the out side of the bag, and through the lanyard on the GPS, before clipping it off on the attachment point.(image d)

( so if the mount should come off in a flip.. i hope that i will find the splash bag, device and mount all hanging by the bungee - crossed fingers:-)

When i land on a beach and want to walk off with the GPS, i just un clip the bungee, remove the bag, device off the mount, clip the bungee to the D ring and walk off..

Step 4: The Test Outcome..

In the case of a roll, (not tested yet) the theory is there is enough space in the base of the bag for the water to run right out again.
I have now tested this splash cover in heavy weather and it worked great - the GPS remained nicely dry and the viewing window did not condensate as my previous ultra tight fitting cover did.
If you need to dry the inside of the bag - check out my "device" for doing he best job(the blue bag in this Instructable is my original splash guard - but it was such a tight fit it was prone to condensation...)