IMHO, the "best" waterproof phone would be the Casio G'zOne Boulder:
It's an older phone, but rugged if you can find one. Use it as a "spare" when you are out in the boat, and have a different phone for other use? Depending on features you want/need, it's easy enough to swap which phone is active at any time...
I think these are all the Verizon rugged phones (past and present) that have been released; all except for the Brigade and the Commando are flip phones.
Casio Hitachi G'zOne Rock C731
Casio Hitachi G'zOne Boulder C711
Casio Hitachi G'zOne Type-S
Casio Hitachi G'zOne Type-V
Casio Exilim C721 Waterproof Camera Phone
Casio G'zOne Brigade [so far the only rugged phone with a qwerty keyboard]
Casio G'zOne Commando [non-flip smartphone]
Motorola Barrage V860
Motorola Adventure V750
The Barrage seems to be too prone to terrible battery life; plus the reviews overall are so mixed that I think you can do much better with just about any of the other rugged phones.
The Adventure also seems to have reviews that are so mixed that I think you should pass on this.
The Brigade and the Exilim are the two Casio phones that seem to have the most mixed reviews of all the Casio phones.
So, basically, the Convoy and the rest of the Casio phones would be the most worthy of your consideration. Regarding the Boulder, I think there are two versions. If memory serves correctly, I think some people found the speakerphone to be deficient. The Boulder was re-issued with a speakerphone on the front outside of the phone. If you're going to buy one on ebay, the catch is that most of the auctions for the Boulder are using stock photos. So I guess you'll have to send the seller a private message to find out which version (s)he's selling. Check out the discussions on the Boulder on howardforums.com for more info about the two versions.
"New Casio Boulder Appears with Front-Facing Speaker"
If you can live without some of the newer G'zone features, there are some good bargains on older G'zone phones that are still available in "new" or "new other" condition like the Type-V, Type-S, and the Boulder.
Check out phonescoop.com, phonearena.com, and cnet.com for what customers think of the Convoy and the Casio phones.