This year I had a trip with Amphibear in Baltic Sea also. As result in some places there is visible galvanic corrosion. Amphibear has frame from AISI316 steel for Mercruiser stern drives. And no matter of zinc anodes on stern drive there are visible marks of white oxide between frame and leg and on small aluminium alloy oil reserve. Obviously I must remove protect and paint these details.

At the same time AISI316 u-bolts fixed to pontoons have no marks of corrosion around them. Situation is different: details are above waterline and pontoon is large compared to steel details. But one difference that matters, all bolts are isolated by plastic washers and bolts itself are surrounded by small piece of plastic water pipe. So these steel parts are totally sealed from aluminium.

If you follow the suggestion to make fastenings always from more passive materials compared to main details you initiate galvanic corrosion. To avoid it, seal it with plastic washers you can buy or even make yourself from sheet of UV proof plastic.