You are able to waterlog trap doors, but not regular doors. This makes for some pretty ugly results, such as a door sitting in a sunken ship.

This can also be used to abuse the ability to breathe underwater. For example, if you're running out of air, you can simply place a door where you are at, and it will make a 2 block high air bubble for you to breathe in, which defeats the purpose of swimming to the top to get air.