That is a interesting point.
We used to have a static caravan, and they certainly do have to have a minimum spacing, something like 5m, as a fire break. You are also not supposed to have combustible materials around the caravan for the same reason.
I would expect that the presence of all the water feels like it makes a difference in the 2 situations, but it certainly would not stop file spreading between boats. In our marina the boats are in grousp of perhaps 20-30 with a promontory separating each group. So fire on one, unchecked, would certainly take out a whole group.