Fresh water seldom permanently harms Jeep radios, mine has been wet enough times that it isn't even funny and it still works fine... once in a while it may need to dry out first though. Besides, you can always get a boat radio enclosure that has a little plastic door that lifts up to access the controls that can be closed in wet weather.
It's the speakers you need to be concerned over but even that is easy if you keep a few things in mind. Most home speakers and many car speakers have cones made from paper. Obviously that's not going to hold up if it gets wet much. If you stick with speakers that have cones made from polypropylene ("poly") and rubber surrounds, they will be pretty impervious to rain or damp weather. Such automotive speakers are actually pretty common.
If you don't have an auxilliary amplifier and are trying to choose between two otherwise similar poly speakers, choose the one with the higher "Sensitivity" rating which is expressed in dB (decibels). A speaker rated at 91 dB of Sensitivity will be noticeably louder than a similar speaker rated at say only 88 dB. That means it takes less power to drive the speaker rated at 91 dB than it would the one rated at 88 dB (for example) and thus the amplifier will have to work less which means it will distort less. Ignore the big print bragging about the speaker's RMS power rating which only tells you how much power it can take without blowing up... not how loudly it plays with any given power amplifier.