navigating with automating a small RC boat

I need some guidance :)

I'm trying to get a small RC boat about 2 ft x 1 ft to be automatic but I'm not familiar marine navigation. The purpose of this is have this RC boat go to a lake or a pond to collect data and return at a specific time. I've replaced the inner of the RC boat with an arduino DUE, motor driver, batteries , GPS module, radio and other sensors. The RC boat has 2 motors that controls the direction and motion.

So far I'm able to calculate the distance from my current position to my destination. I am able to calculate the course (degrees from mag north) from current to destination position and I know my boats heading compare to mag north ( using compass).

I have yet to test this in real waters yet but I've tested the heading and course outside. I am able to get myself pointed in the correct direction of my destination and traveling to the destination on foot.

Anyways, what I have now is that the RC boat will rotate itself until it faces the correct direction of the destination and go forward until the distance between the two is at some margin.

So when the boat is turning itself I want to give it a good margin of error perhaps like 5 or 10 degrees so the boat won't continuously adjust itself trying to match exactly the course direction but I want this margin to decrease as it approaches the destination so that it will be as close as possible to the destination but within an acceptable drift zone. Right now I'm thinking of making the margin of error as a function of the distance I am from destination. Not sure if this is a good way to do it or not.

If you think there's other problems I should give my attention to leave a comment or link me to similar projects done!

Thanks!

http://en.wikipedia.org/wiki/Voith_Schneider_Propeller

you can get this for model boats too!