If you will crash on land or a
buoy in all 3 directions, then you must take the
direction with the longest straight path through
water before crashing. If that is equal in 2 or 3
directions, then you choose one of these directions.
That's what the rules say. They only restrict movement options if one or more of them would result in a CRASH. Having to back up a space because there's a boat in your end space isn't a crash. So I'd say you'd be fine going STARBOARD (gotta use the lingo for a boat game) moving your 4 spaces, and then backing up a space because of the other boat.
I'm curious how they have it programmed on Mastermoves, but under a strict reading of the rules I think my take is correct.
Another interesting possiblitity is that you roll a 4 and have:
-4 spaces each occupied by other boats fore.
-an island immediately starboard
-an island immediately to port.
You'd stay in the same hex, take no damage, and move on to the next player