Drowning is a mechanic in CivRealms 2.0. If you try to swim in water more than 2 blocks deep, while carrying more than 8 full stacks of items, you will be pulled downward into the water faster than you can swim up, and will eventually drown.
This effect only applies when the amount of water below your feet is 2+ blocks. So if you are on the floor of a lake or ocean, you can hop up a 1 high ledge at a time, even if you are at the bottom of a very deep section of water. This will usually save you if you accidentally jump into a natural body of water near a shore.
"Full stacks" refers to however many items of a type can be stored in a slot. So a single piece of armor is a "full stack", so is 64 normal blocks, 16 pearls, etc. 8 pearls is half a stack, so is 32 normal blocks. Examples of "stacks" are shown in the image.
Note that air bubble count will persist through rejoins, so drowning can't be evaded through constantly quitting and rejoining the server.