The Water Survival skill is used passively it allows you to access surfaces and areas that might otherwise be inaccessible due to them being submerged in water.
This skill is the only survival skill that all players start with. This allows players to access tiles with fish and such on them …
Water survival is used to access tiles that are partially or fully underwater, such as rivers or flooded caves. Someone without the proper training could easily drown or not recognise the dangers such as bogs and floods without this skill so that is why it is required.
See the image below for the list of terrains Water survival is needed for.
You can also use Air Survival to fly over non submerged water tiles. However Air Survival will not work for flooded rooms.
Additional Information
As with all of the survival skills Water Survival can be obtained from finding items that contain the Water Survival bonus on them.
Water Survival can also be acquired through mounting any of the following pets:
You can also consume certain food items and/or potions to acquire the Water Survival skill.
All players start with the Water Survival skill so the only reason you may have for raising it would be to access dungeons that have specific survival requirements or for finding items to equip a pet so it can access a Water tile.