The problem is none of those would last more then a month outdoors. Especially in a marine environment (I plan on using this on a boat).
This claims to be "weatherproof"...
I believe it'll work like mine, where the speaker is "off" if you simply turn the volume all the way down. I've had mine outside for years, but admittedly in a partially sheltered area. No problems. I bought mine at RadioShack, but the above one looks bigger, better more weatherproof...
Good luck, and have fun on that boat; sounds like a nice one if you've got 3 or 4 pairs of speakers!