More weight = more force. Think of being at the pool when the 5 year old weighing 80 LB's makes a cannonball vrs the 15 year old that weighs 150. Same idea with the boats. When they are empty there isn't enough weight and force to make a bigger splash. But even load it half way and there's much more force and weight to hit the water with.
Not only that the empty boats also displace far less water than loaded boats. Think of being in the bath and Archimedes. You never fill the bath the full way. Only part way. But if your significant other is joining you in the bath, you need to fill it even less. Same things with these boats. That splash is just the amount of water that's being displaced to keep them afloat. The deeper into the water the boat sinks at the start, the more splash potential it has.
