You cannot claim international immunity on the high seas if you are only high in the St. Johns River. Boating while intoxicated violates the no wake zone of justice. Walking a straight line on a boat is difficult, but you may be walking the plank if you are charged with BWI.
Eye tests and breathalyzers do not require dry land for a field sobriety test, neither does the ability to recite the alphabet backwards. Failure on those drills could land you with a BWI arrest on probable cause. The best way to avoid such an arrest is not to be intoxicated while boating and follow the rules of the water; however, there may be another factor to consider to prevent drawing attention of the authorities.
You may not have consumed any alcohol, but if you have been out in the sun, boating all day without sufficient replenishment of fluids, dehydration could make you exhibit unstable symptoms, enough to make an officer suspicious.
As with any situation, use good judgment. If you have been arrested for BWI, find out what your options are. You may have a defensible case.