You'll do well with vertically polarized antennas very close to, or over salt water. Even if set back 20 or 30 ft - you will lose a very large percentage of the salt water enhancement. It really needs to be at the water's edge if at all possible. I spent 5 years on the edge of salt water in Key West and verticals mounted on the dock over salt water (with one elevated radial per band) greatly outperformed the same vertical set back 30ft from the water's edge.
100w and a multi band vertical and you'll have lots of fun. Do a Google search for K2KW and read about his elevated radials - I prefer them to radials in the water. They worked much better for me elevated. AA7JV on the other hand (PT0S) prefers radials in the salt water but uses remote tuners to adjust for tidal changes (which will change antenna resonance).http://www.n2nl.net/?page_id=40
describes the dock mounted antennas I used while in Key West.
Thanks so much for your advice. You had a very nice setup in Key West.
Since I will be only on the island for 13 days I need something a bit more simple that I can assemble / disassemble quickly myself.
Also, I am not sure if the hotel allows elevated radials on the beach - they did confirm via email that vertical ham antenna would be ok.
Thanks also for the K2KW tips - I found this page http://www.k2kw.com/verticals/learning.html
that had a lot of practical information about effects of salt water on conductivity of guy wires, insulation etc.
However if the radials are under water it sounds like remote tuner would be needed to adjust for impedance variation. Both Flex3000 and Elecraft KX3 have built-in antenna tuners - not sure if they would be able to handle the impedance changes? Signal loss in coax is another factor to consider.
Based on information I got from the hotel the beach is about 70 feet from the building. Since I am quite limited with luggage weight I need to consider also the weight of coaxial cable and possible remote tuner. Weight of 100ft RG-8 vs. RG-58 is 13.7 lbs vs. 2.6 lbs so better coax would add significant weight (but also reduce losses).