YMMV on that one. Their armor is outright stated at the beginning of the game to be very very thin. If a palm-sized rock is enough to knock a guy out (despite his helmet) the armor couldn't be very heavy. Its possible that he still could have drowned with the accumulated weight, but not likely.
Still an awful dick move on them, though, especially when used as humor.
Compared to historical protests against tax collectors, this Troper viewed it as light.
It's doubtful Yuri would've just left him there. He either fished the guy out himself or made sure his buddies were able to do it. Assuming of course the armor was indeed too heavy.
Yay! You've destroyed all the Blastia! Isn't that great guy with a blastia for a heart . . . Oh Crap. Just as he was starting to not be a Death Seeker too.
Which leads to the Fridge Logic: the ending shows that he did, in fact, live. How?
This is sort-of addressed in the game; his blastia is unconventional; it doesn't run on aer, but on his own lifeforce. As such, presumably it was not converted.
There's also a scene you can get in Dahngrest where he's told to have the party's blastia expert take a look at it; assuming he followed this advice, it's possible that they were able to get a spirit to power the blastia.
So, we put an end to superbeings powering the worlds machines... so that another superbeing can power a machine. What.
But this time, the superbeing powering the machines in question won't destroy the world.