Okay, I'm probably gonna get shouted at, but here I have an idea. FISH and BIRDs. They fly/swim around in a random pattern, and can be used to add realism to models or cities and things. They could be attracted to say, SEED, which could turn to PLNT if dropped in WATR, or PLNT, and FISH could be attracted to BAIT or PLNT. They could die (i.e. disappear) if the water temperature or the temperature of something they hit was too hot/cold, too much or too little pressure... FISH could die if the WATR is sparked... I dunno.

A.I. they;d just swim aimlessly. If you saw my Seaside City, you might have seen the fisherman, I could put FISH into the water, and a bit of BAIT on the end of the fisherman's line...

It would heat and/or cool the water a _lot_.