I'd never gone underwater, though thinking about it, now essentials is installed I haven't been able to get out of Lava and survive like I did in the past.
The drowning things seems to be hit and miss. I've had both things, where I can drown and where I can't. And this was during the first few days of the server before you went down the tried and tested path of ruining it with installed mods. Might be latency related or sommat.
there are no monsters
when falling I still got the damage, ie a big fall could kill, but small fall damage would recover
with water, I could walk about for ages underwater, I'd be getting 1/2 a heart damage but also recovering by 1/2 a heart at the same rate
it's not a problem though, I like it slightly more dangerous
