That is the same as BOSS car audio. That is just a "marine" version that some manufacturers make. The only thing that makes them "marine" is the cones are made out of polyurethane and the grills are plastic. However many quality speakers are made out of polyurethane anyway. The metal grills may rust if they are exposed to water but they can easily be painted black again with a spray can. For decent speakers you can look at Infinity, JL Audio, Polk Audio, or MB Quartz. Expect to pay around $100 a pair on ebay. Amps are similar. High end amps are Phoenix Gold, PPI, JL Audio, Soundstream, Memphis. You get what you pay for.
You still should try to protect your stereo equipment from water as much as possible. Wipe the boat down and put a cover and dehumidifier if your boat is going to be exposed to the elements.