One thing John Lordan mentioned and showed on his video is how close Michael would've been to
Hermosa Beach Pier, which extends into the Pacific Ocean.
Instead of a late night swim, could Michael have gone to the pier instead, and somehow slipped and fell into the water? This would explain why none of his possessions was found because they all would've abruptly gone with him into the water. But it wouldn't explain why nothing washed up on shore, though I don't know what the currents are like over there.
It also doesn't explain how this could've happened without any witnesses, though maybe there were few to no people on the pier—or at least at the spot on the pier he could've fallen from.
One other thing I noted in the episode: an investigator said he doesn't believe Michael went for a late night swim, left his clothes and possessions on the beach (as he had at other times), and that they were subsequently stolen because the presumed thief would've turned on Michael's phone. Since the phone never pinged anywhere after a certain time, it was never turned back on, and therefore it wasn't stolen.
I think the flaw in this argument is that it assumes Michael's phone was working properly, and that Michael turned it off intentionally, or the battery died. But what if the phone had an unknown technical problem? In that case, a presumed thief wouldn't have been able to turn it back on in the moment, but they still could've taken it for parts or just discarded it later after realizing there was no use in trying to power it on.