When I was running in salt the common practice was to blast everything with fresh water after every day. To find a mounting place that wouldn't get blasted with fresh water it probably wouldn't get enough circulation for cooling.
To answer Rod's question. Yes, they actually do make marine amplifiers. They just epoxy coat the circuit boards and plate the connections a little better. Not much different from a technology standpoint, as Adam mentioned, maybe a couple years behind.
In the mainstream marketplace there is not a huge demand for high output marine amplifiers. In reality it's a very small niche market. I believe that Clarion, JL, and Infinity all make marine specific amps.
Most of today's (new school) amps are sourced from China with less than ideal fabrication standards. On the flip side they are fairly cheap. I am using a couple HiFonics car amps right now, they are cheap and seem to do a decent job. They are somewhat disposable, probably not a bad choice if you budget a replacement every 3 or 4 years. The car amp should live through the humidity.