Given the desired heading it should be easy enough to calculate an error term in the range -180 to +180.
If simple proportional control gives too much of a dead band (error term too low to cause a correction but too high to be on course) you can change to a PI control. If the heading oscillates, you can go to PID control.
One thing you should be aware of is that your course (direction of travel) will not necessarily be the same as your heading (direction the boat is pointing). If there is a side force (current or wind) then the boat will drift sideways relative to the heading.