NOT FOR YOUNGER PLAYERS:
Blood traces can be seen on a block when any player was damaged (except stupid people shooting you or spading you but only if you shoot griefers). Blood traces lasts only 30 secs and changes the water colour to red if any blood goes to a single water block.
Blood may have different impacts:
SMG shots gives little blood splashes
Shotgun gives a MASSIVE gulging blood
Rifles gives the opposite splash (like you shoot infront and blood goes back)
There's almost every ideas that are already used, so there's less to think of.