I have to replace my compressor driven-12v horn. I would like to get
as much
"ship" sound as I can for 15 amps of 12volt. Any suggestions?

Your requirements don't make sense. An electric horn that runs from
12VDC and draws only 15 amps is going to be significantly LESS loud
than an air horn--even those little plastic air horns with on-demand
vane compressor (5psi) are louder than any electric horn. What you need
is a 12V piston-type compressor, small tank and proper air horn. For a
nice deep tone the Grover 33" and 37.5" organtone horns are a good
choice and they're affordable (about $90 per horn). They are truck
horns, but all brass and chromed so they will hold up fine on at sea.
Look at http://www.groverairhorns.com for horns and compressor setups.
That site is for a popular reseller, for the company site see
groverproducts.com.

If you absolutely need the loudest electric horn money can buy, the
only option is finding a used Federal Signal "Traffic Blaster". But
don't expect air-horn class loudness, I didn't even need earmuffs when
I tested these horns.

-Adam