Here comes from someone who has had different power and sail boats, lives in PR, and has a house close to the water...
In my house I have outdoor Advent's (no longer made), been through two tropical storms, many rains, and 8 yrs. working. Back then Costco used to sell them at $98.00/pr. They are still going strong, no noticeable SQ difference from when they were new, and if you clean them every month or so they will not corrode...Now, the same cannot be said for the wall brackets that came with them.
In our beach house we have Paradigm outdoor speakers, and they been there for about two years, and no problems. It is very important to rinse them with potable water whenever you can so the salt comes loose.
In the boats I have had many different brands of marine speakers (jennsen, sony, etc), and all have had to be changed after 3-4 yrs of service. But, when you are close to salt water, even Hastelloy C will corrode.