Experiment
You could try a wee experiment. Fly a plane with guns that use a round with the watereffect fx_bltwtr_m. Shoot at some water and see what the water hit effect looks like.
Then take out what looks to be the stock bullet water medium effect (the shorter one) and go shoot some more water, see if the effect has changed. Not at my CFS comp at the moment, can't take a quick looksee to see if for example the Hispano 20mm would do the trick.