I have a friend that is heavy into racing sailboats (1-2 million dollar 40-80 foot ones). He has a few pictures from the top of the main mast shooting down... all I will say is this:
1. highly doubtful anywhere you pay to sail will they let you go up to the top of the main mast.
2. hope you are not sensitive to motion sickness.. lol
3. if you can get up there, pay your insusrance before going. Hitting the water from 80 feet up is like hitting the asphalt from 7 stories high... not fun.
That said, I *do* hope you have no rain but lots of clouds. Nothing worse than trying to take shots on a really bright and highly contrasty day.