Beginning is the scariest part in terms of visual, darkness, creepy music. When it's dark you will drop off down something no where near Splash Mountain (you are in big boat with no restraints) but I think the fear kicks in with the unknown. Because it's dark you don't know how far it will go, let him know it's not far. Then you land in "open sea" area with pirate ship and cannon explosions and sometimes splashes. Some of these blasts can get folks on right side wet as well. All about timing.
Yes you absolutely may get wet, there is even a sign out front that says so, it seems to be just random who it gets. We have gotten in boats with standing water on floor and wet seats. We have gotten off with shirts and shorts wet. Nothing soaking like Splash Mountain but enough someone would notice we are wet. Take a baggie if he truly is concerned or put it away.
Disney doesn't post signs lightly, be forewarned.