The 'marine' CB antennas you'll find (typically mfg. by shakespeare) are 8' "no ground plane" antennae and do not work worth sheebotz. Not only that, but they require the 'marine mount' ($15-$40 dollars depending on material/quality) to affix to your 'toon.
You won't have a problem with a whip or whatever as long as the ground is connected firmly to the aluminum on the pontoon and that all the aluminum subparts of the vessel (decking / railing / platforms / the toons themselves) electrically attached to each other.
HERE's what I'd do, if you don't go under too many bridges. I'd mount an Antron 99 or equivalent to that puppy. Then you'd have blanket coverage. Use stainless u-bolts and affix the antenna firmly to a good stout piece of a vertical grab piece of the railing on the vessel.
Then you will have blanket coverage. The Antron (~18') will be cheaper than the 8' "no ground plane" Shakespeare 'marine' CB antenna.
To finish off the installation, use a quality Marine grade Mini-8 type of coax mfg. by GoldLine or another respected manufacturer. Get the white stuff, not the black and it will not only look like it was installed by a professional, but will give you several years of trouble-free service.
One thing to note is that the Copper in Fiberglass Reinforced Plastic types of antennas see a lot of harsh weather conditions on a vessel and that tends to make the Fiberglass get brittle faster than normal. Wear thick gloves when you work on or remove the antennas to prevent splinters to the hand.
Have at it.
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Buttfuzz in the Mobile
SuperStar Gr, KLV203, Wilson5K, 575-M6
Buttfuzz at the Base
Galaxy Class StarShip, A99wGPK, D104-M6B
Buttfuzz Built Deuce and a Quarter</p>