A number of issues have been raised here
that deserve some further comment.

Regarding RF coupling to the standing
rigging, don't be overly concerned. You
will be coupled to the rigging no matter
where you place the antenna. Some
geometries will doubtless be worse than
others, but it will be difficult to
predict in advance. Moreover, such
coupling is not necessarily a bad thing.
It is just a difficult thing to model
and thus more of an unknown.

Regarding antenna length, more is not
necessarily better and may be worse! But
whether worse or better, changing the
length of an antenna may make it
different. For example, if you are
crossing oceans and want reliable skip
communication over great distances, you
want low radiation angles. A
quarter-wave or 5/8-wave vertical will
be your best choice. That would be about
16 feet in length at 14 MHz. Make your
antenna 32 feet long and you have a
half-wave vertical with very little
low-angle radiation at 14 MHz, but at 7
MHz and below, low-angle radiation will
be plentiful.

Which is better depends on your
objectives. Operating near the coast,
you may find that higher radiation
angles produce shorter skip zones to
your advantage.

Were your boat fiberglass or wood
instead of steel, it is possible that a
horizontal antenna laid on the deck
would outperform any vertical antenna
for high radiation angle communications
with a range of say 400 miles. Think
about maintaining solid ssb contact with
boats scattered throughout the Bahamas,
for example. With vertical antennas such
a task would be quite difficult.

Start out your planning with some
consideration of which distances are
most important. From that, move to which
frequencies and radiation angles provide
the appropriate skip zones to achieve
those distances, and from there,
consider antenna options that further
those objectives.

Good luck

Chuck