"Most monsters, even with their high strength scores, have the Athletics/Acrobatics checks of a 1st level commoner."
I'm not sure where the idea that the typical monster (that isn't low level) is as easy to grapple as a commoner comes from. The defender chooses whether to use Str or Dex. Most creatures--especially after the lowest CRs--have a good modifier in at least one of those stats. We're looking at a +3 or higher for most of them, while a commoner has a +0. There is no penalty for not having proficiency, you just don't get the bonus. So a high Strength, in and of itself, does set you above anyone else who lacks proficiency. A low-level PC trained in Athletics has a better modifier, but that's part of the point of being trained. And Acrobatics only works for defense--so your low-level character needs to have a good Str and be trained in Athletics to really be outshining most opponents.
I'll admit that most people in our group didn't really notice the usefulness of grapple, so it's nice to see the ideas on here. The ability to keep them prone is pretty nice at giving the feel of "restraining" them even without imposing the actual condition--and throwing them off of ledges is always a neat trick.
I'd rule that a creature that is forced underwater, rather than intentionally holding its breath and going under, would start drowning immediately. It might be more effective to kill certain foes that way, but it's pretty situational and I'm going to bet the numbers favor just stabbing them more often than not.
