The thing that pops up when you are hit is called a HUD image, and you can add your own HUD images in any size or manner you want using, you guessed it, FPI scripts. Check out the DOORUSE.FPI to see how an image was loaded, displayed and sustained when the player gets close to a door. You can use the same to create your own 2D image (or animation of them), and use the native alpha data of the TGA format you add semi-transparency. You can then play a sound effect over this visual, and change the players footfall sound effects (from the soundset (see manual)) to sound like splashing. Have fun!
"We are the knights who say...eki eki eki fatang loopzoing, zanziga....ni"