As per title. Everything you do underwater, from attacking to healing to gathering resource nodes, takes half the time to complete than on land.
As a Mesmer for example, Mind Blast (our Downed 1 ability) is 2x as fast underwater than on land, which is part of the reason why our underwater Downed seems so powerful. Ether Feast likewise casts much faster underwater than on land.
A few patches ago Anet “fixed” a bug that made Mantras take half the time to channel underwater. That was actually not a bug, it is because of this bigger issue. What Anet should do is fix this bug and remember to revert the Mantra band-aid “fix”.
If Anet is going to balance underwater combat, this bug would be a very good place to start.