I have an old garmin GPS that I am not using any more and I am wondering if I would be able to use it on the water. Its a gps that is designed to go in your car, but I would want to type in the coordinates of the boat launch, just in case I get lost, so that I would be able to find my way back. Would the gps recognize that I am in a boat instead of a car? I am just trying to save some money by using this one.  

There are no highways on the water to give you turn by turn directions.

You can set the GPS for off road navigation. You store a waypoint for your launch location and when you want to return, just set it go to the launch point. It will just point you in a straight line back to the launch. You'll have to have it connected to a power source because the batteries in car units won't power it for long. Car units are also not waterproof so you'll have to protect it from rain and getting wet.  It will work fine for what you want to do.

I used an old Garmin made for a car on my boat.  I bought a Garmin chip (I believe it was Garmin Inland Lakes) and inserted it in my GPS.  I then selected it to use the marine chip and everything worked well.  It is not as good as a true marine GPS, but worked well enough.  I also used the car mount and stuck it to my console.  Give it a try!