Simply spraying the hull with water to rinse it isn't going to cause problems as long as you let the water dry before applying the paint. It isn't like the bottom is all that porous or easily saturated with water.
If just the last coat of paint was chipping, it isn't because you rinsed the hull. If the paint was going to flake off because the hull was rinsed, ALL of the paint would be chipping or blistering, not just the last layer.
I'm not as sure. The chips were over prior
years paint. When the chip appears, there is still one (or more) coats of paint underneath - not
bare gelcoat. Furthermore, most of the chips were near the waterline - on that portion that gets (intentionally or inadvertently) more paint than the rest of the hull. This is not to argue with your point, but better describe my own.
I suspect the rinse left moisture on the old ablative (and relatively porous) paint. When the new coat was applied it dries as expected, but did not achieve adequate adhesion
to the surface, particularly in those spots with some amount of moisture still present though not visibly detectable. To your question, why one layer and not all
? My explanation is that the prior year's paint achieved proper adhesion, the last layer could not because of small amounts of moisture within the base ablative paint caused by the water rinse the day prior to painting. What say you to that: reasonable possibility, or pure bunk?