VERTICAL ANTENNAS over SALTWATER are extremely effective,
and probably the best choice if limited height is available.
You will need some sort of "connection" to the salt water with a
"fair amount" of surface area. I have heard of dropping several
bare copper wires into the water, or a metal plate connected by wire.
I believe one of the ZL7 operations used a galvanized bucket with
a weight in the bottom.
The MFJ-1792/1793 verticals for 80/40 and 80/40/20 look very
promising for the low bands. They consist of a full quarter wavelength
radiator on 40M with a 40M trap and a capacity hat for top loading
on 80M. The 1793 has a full size 20M element in parallel to the 40M
radiator. I see no reason this side element could not be lengthened
to provide 30 meter coverage instead of 20.
For the high bands, you can have your choice of numerous
multi-band quarter wavelength verticals from Cushcraft, Hy-gain,
MFJ, and several others. A single ALL-BAND (10 -80) quarter wave
vertical is another possibility, but they will have reduced bandwidth and
somewhat lower efficiency on the low bands.
Or, the so-called half-wave verticals might also be of interest for the
higher bands and require no connection to the "ground (or saltwater)".
Several years ago, K1KI took second place in the CQ WW SSB
DX contest from PJ9 using a Hy-Gain 18AVQ overlooking saltwater
and a 160M inverted vee. First place went to a PJ2 with a TH6 and
2 element 40M beam on a 70 ft tower.
73, Tom - N4KG