Lots of seaplanes are buying handheld marine radios and just plugging
into their aviation antennaes without any problems. The marine hailing
frequency (I forget which channel that is), is monitored by allmost all
boats up and down the inland passage from Washington to Alaska and if
you have trouble this is a good frequency to be on. It is also
monitored by the coast guard. The aviation antennae gives better
results than the rubber ducky handheld in most planes, I can tell you
that. The newer audio panels have music input jacks that you can plug
the marine radio into and listen (but not talk) on your aviation
headset. Nice setup at low cost.

From what I have heard the FCC isn't going to permit an aviation/marine
handheld to be manufactured. Just not in the cards from what I have
heard as Oshkosh.

Of course a built in marine radio into the panel of a seaplane is a
nice feature too. I don't know if there is a marine antennae that is
specifically made for installation on an airplane or not. Might be
interesteing to google around for one.