In the older version of Minecraft, water would destroy any Minecart or tracks it came into contact with. As of the Minecraft Caves & Cliffs Part 1 major update, that is no longer the case. Rails can be waterlogged, and Minecarts can ride along with them under the sea, but there are four things that everyone making an underwater Minecart voyage should know.
Minecarts are no longer destroyed by flowing water
If flowing water hits a Minecart track of any kind, the track will no longer be destroyed. Instead, it will behave as waterlogged until the flowing water is picked up, blocked off, or redirected.
Minecarts have a speed nerf underwater
Minecarts move at a much slower pace under the water. This means not only will you move at a slower pace, but you will need to use powered rails at closer intervals as well.
Minecarts have no air source
You can still drown while in a Minecart. If you are riding in an underwater Minecart, it is good to bring a respiration helmet and/or a Potion of Water Breathing.
Detector Rails are your friend
Detector Rails are the easiest way to power Powered Rails underwater, as water will destroy Redstone and Redstone torches if water touches them.
