As far as enclosed marine speakers, both the B.o.s.e 251 and the Energy Tempest II can be had new on Ebay.
Marine speakers are designed for use in the pilothouse, cabin, and outside dec areas, and to resist a corrosive environment for some period of time. I believe both those speakers are warranted to work for 5-years while being subjected to a salt spray and torrential rains. Since almost nothing on a boat can withstand 5 years exposed to a marine environment, I’m a little skeptical about the manf’s claims. I guarantee even tin coated copper speaker wire won’t last that long unless all terminal connections are sealed.
Bose also makes the model 151 and 161 enclosed speakers which are also sold thru boat outfitters. Only the model 131 requires flush mounting.
Sorry, can’t speak to the quality of either the B.o.s.e or Energy products. Fwiw, the Bose may not have a good reputation on the audiophile forums, but that 251 speaker does have a good rep within the boating community. Keep in mind boats are not the best place for critical listening.