Gotta agree with the 'Duck....
One thing to remember, with the antenna placement you describe a quality 5 watt handheld will receive just as well as the 25 watt fixed mount. However, the 25 watt output on transmit will definitley improve range over the handheld, but you will be amazed at the performance of a handheld utilizing an external antenna.
The determining factor in your proposed set-up? The quality, and hence the loss factor, of the coax cable you intend to use. Do not scrimp here, as cheaper cable can have double or triple the loss of good quality marine grade RG-8 or highr loss RG-58 cable.
Frankly, if you are doing all your sailing on Winni I think that a quality handheld with external antenna option coupled with a portable stereo system is your best option. Not only does it give you many portability options, but when you leave the boat you can take your electronics with you and cheat the thieves out there!