When you talk about making a vehicle (or a boat in this case) Autonomous, the algorithm part comes later.
The first question should be that is the boat Drive-by-wire enabled? Meaning, can we maneuver the boat purely by computer commands?
If not, that has to be the first step here. Think of this as converting a manual vehicle into Autonomous one, so essentially the requirement you post is NOT just a software one, but in fact a Mechatronic one.
So in order to achieve what you propose, one would require a team of mechanical, electronic and embedded software developers which would first make the boat drive by wire and then perhaps work on the path planning algorithm you desire.
The point I'm trying to make here is this ain't a hobbyist side project which can be done in $150-$200, expect this to cost you more than $3000 any given day.