A stretch back doing MH with NW3, I froze a Titan and it slid into a lake...

... and I thought it was so cool that it sank and then bobbed back up the surface.
Got me curious though, so I used a redeemer to "nudge" a Titan into a lake... and not surprisingly it couldn't swim. Thing is though it didn't drown, it just sank to the bottom and stayed there. I watched it for about 10 minutes, dodging the occasional boulder when it saw me looking down.
Nothing.
Tried this another time with a Queen. Also didn't drown.
Never tried a Brute, but the Behemoths don't drown either.
Why don't they drown?