The only pellet that I can think of that is significantly water soluble are Palco's Eco BBs:
I can't say anything about their ballistic properties or loading reliability as I haven't tried them myself though. However they did dissolve into loose powder in minutes with water immersion. They're probably the fastest degrading pellet available. I saw them at ShotShow several years ago. Their fetching marketing ladies were dropping them into containers of water to show that they broke down in minutes.
They might not be the best alternative to tolerating inert pellets in the sand though. IRC their outer shell used a sugar as a binder which may attract ants or promote fungus growths in bulk quantities. I really can't see how rigid bits of plastic would be a problem in an indoor pball field which gets saturated with non evaporating pball muck. I think they have to scoop up the sand and properly dispose of it once or twice a year anyways.