Notes of Caution: make sure you are invisible, and are affected by underwater breathing. If you are a fey, be very careful - make sure you have to HP to withstand losing HP on ticks, as you may lose a couple hundred hp per tick and die due to water damage. Be careful of the aggro mob after going down, flee down or back up from it. Most of the region is no-magic, so be careful about your HP. Don't wear any equipment that you don't want to lose - if you die, the entrance will teleport somewhere else in the Decara ocean and you may have a very hard time finding it again.
Update: Due to changes on how drowning works, it is now imperative to have underwater breathing, or bubble cluster or otolithic growth otherwise you will die after couple of gulps of water.