It all depends on the money you want to spend.
And while moisture use to be an issue, most mid-level and up speakers will be made of a moisture resistant material.
The bigger issue is sun exposure and heat. There are speakers that use rubber surrounds, and others use a treated foam, and still others. If your speakers will be placed where they receive direct sunlight while you're out - you want to spend enough to make sure that you get a good rubber or silicone surround. Some marine products will have a UV coating on the surround or use a UV resistant material so that they don't degrade in the sunlight. The direct sunlight is a much more harsh environment than a wet one (from the front side of the speaker anyway, keep the moisture away from the back and solder your wire directly to the terminals).
For the price, you can get marine audio "packages" (head/source unit & speakers) from places like Crutchfield that aren't bad at all. If you're really looking for a deal, look at SonicElectronix.com ... I've purchased from them several times and have been happy. For many years I was involved in psycho high-end competition car audio ... the show stuff you'd see at CES in Vegas ... Feel free to message me with questions. A boat is a very difficult environment to expect any real sound quality, it's mainly for ambient music. Alpine has always made very reliable products - and they specialize in mobile audio period. You see Sony alarm clocks, tv's, stereos, yada yada ... Alpine puts all R&D money on mobile audio/video, they don't do any home electronics at all. At a consumer level, I would highly recommend Alpine.