Found this in same drowned deciduous tree as a bunch of new-to-me teneb*rionid larvae
and a weird-looking Pityo*bius clic*k beetle larva
. I'm hoping all will mature so they can be further IDed and photographed. I also found some uropo*did mit*es that I have yet to process and post.
This larval was quite plump and showed very little motion. I suspected (wrongly, see below comment) that it was either parasitized or ready to pupate. My closest guess was beet*le family Bostrich*idae.