Yeah it could be the water on the map; San Andreas has this problem too with my boats. Try it on one of the ocean slate maps and see if it works. If it doesn't, you will have to find a balance between density (usually between 0.2 to 0.3), mass (values over 100 might cause problems), and drag (which prevents the boat from bouncing around on the water, but can also decrease yaw - turning - depending on density and mass). Hope that helps.
