First off, grats on the new forum avatar. May you have another 1000 posts without incurring mod wrath.
Secondly, thanks for the heads up. I figured that might have been what was happening. I'd seen groups vanish before and figured that was them despawning, but I didn't think they'd respawn in a slightly different spot moments later.
Of note, they were not, iirc, near a harbor. Nor were they on path to one. They had turned into the gulf, but appeared to be turning to leave it again when they despawned and respawned on top of me.