Check this site out, it contains top quality marine audio and provide a wealth of information answering most of the topics listed here.
https://www.rocktheboatmarinestereo.com/
I purchase a JVC head unit that is waterproof not potted. I also purchased DC Gold speakers that are submergeable. 9 years of heavy metal both are still going strong!
Another note: I replace the OEM Babb speakers which were 29 years old with the DC Gold, as DC Gold bought out Babb. Babb were also waterproof.
These products are available on rock the boat marine. Feel free the hail me if you want to check out the audio system!