This is done rather hackishly, but Minetest doesn't
seem to offer a sane way to register fluid buoyancy, or
even a reasonable hack to use negative gravity for this.
Instead, just push player upward with velocity impulses
every so often.
This is helpful gameplay-wise to make chatting while
swimming easier for players not using auto-forward, or
to prevent players from drowning and losing their stuff
while swimming long-distance and dropping network
connection and their in-game character waiting in the
water for the timeout. Instead, players will return to the
surface and remain where they can breathe until the
player can take control again.
