Stand your ground law in FL. Call the cops after you defended yourself from the threat.You are ignoring the difference between rain and someone shooting water in your face. Rain is an act of god, someone shooting water in your face is intentional. It only becomes a question of whether you would be charged with assault or battery... but regardless of that, the person could sue you for it and at some point they would also sue Disney for failing to take necessary steps to protect guests from the unwelcome assaults (like not allowing water guns).
You also have to realize this is Florida and people have jumped other people and beat the hell out of them for a lot less than getting shot with a squirt gun.