It's possible to build a rail line underwater in #Minecraft. It's just annoying. The launcher must be in a dry space so that the button will stick to the redstone block. The minecart travels slowly regardless of how much powered rail it goes across. Seriously, I made a line that was all powered rail and I could have swum just as fast. For long distance travel, sprint swimming is still the best option, especially if I can pass by the occasional dolphin.

Here's the photos of the two #Minecraft underwater rail lines I built as experiments. Yes, that's an ocean monument next to them. That's where I live.