I need a CB for my Jeep but also should have a Marine radio for offshore use. What's the difference? How far offshore will a 5 watt hand held work for?

More accurate info will follow, but isn't the CB "Citizens band" radio. Where a marine VHF is "???" (ok I don't now that one) I believe these are not compatable at all and therefore you need both.

Yep, what he said.
BTW, does anyone use CBs anymore?

The marine radio has a much greater range. It also has frequencies which are specific to the marine enviroment. When my boat was sinking the Coast Guard on the far side of Lake Michigan handled the call. They were proably 80 miles away. Try that with a CB and nobody will respond.

Hmmm. So I guess I need both. When we do Jeep runs in the desert we run CB radios for communication.

The good news about a marine radio vs. CB vs. cell phone, is that EVERYONE listens to the marine radio, CG channel. If you are sinking or whatever, you'll have a lot of folks trying to help. The other two, only the guy you call.....

I don't think it is technically legal to use a VHF radio for other than marine use.

VHF and cell for water.
CB for truck, works good if something like a cover starts flopping, a trucker will let you know.

the cb is good for talking among friends or truckers down the highway. For the ocean I would only use a VHF radio. The Coast Guard may listen in on the cb but I would not take that chance

VHF = Very High Frequency.

And agree with VHF = Water. Almost all (all?) marine VHF radios have channel 16 override, so if you call for help on 16, every VHF within range will switch to channel 16 and hear your distress call.