It has been my experience, sailing a 43 foot cat, that output power higher than 25 watts or so is not needed. When a 2006 hurricane damaged just about everything on the boat, the sealed (?) 1/4 wave marine VHF antenna was replaced with a run-of-the-mill Larsen NMO150 using their ground plane base adapter. It was relocated about 3 foot higher, and somewhat clearer of the mast. The difference was night, and day. However, I'd say that 99.9% of the time, a handheld is all that's needed to get the attention of the drawbridge operator, or any nearby boat (< a mile).
What you really need for emergencies, is a channelized SSB marine HF radio. That is now an Icom IC-M802. It is loaded into the port mast stay using an AT-140. The starboard stay supports the IC706MkIIg, and AH-4. Best of both worlds!
Alan Applegate, KØBG