Disney security's hands are literally tied. The only thing they have at their disposal is a citizen's arrest, and the last thing they want is their security guards physically detaining guests. They also don't want to call the police for every petty offense, and it sounds like at most they had a suspicion of trespassing. If the could even prove he entered the water intentionally, what would the result be? They also don't want a situation where guests are followed through the park by security, attracting the attention of other guests either to try to compel them to leave or wait for the police to arrive. And how are they going to force a guest to hand over the ticket or ID? In a more controlled situation like a guest causing an issue in the hotel when the guest is in their room, it makes more sense to involve the police. But on the platform of a ride it would have to be something much more serious.
And, if they start kicking people out of the park for this, why not start kicking people out of the park for line cutting again. And that includes joining your group already in line. What about when your kids are swinging their arms in line and hit someone, or fighting and accidentally shove another guest, or aim their bubble wand in someone's eyes? What about when a ride stops became someone drops something on the tracks or because someone refuses to unload until they take a selfie and the ride times out and has to close. Be careful when you call for people to be expelled from the parks that you've never done something negative to impact the park or someone else's experience as well.