please help detonated piston

I have a question if the wrong plugs could do this kind of damage. 96 200 saltwater had been running good but gave it to my mechanic for maintenance, plugs, impeller, lower unit fuel sep, etc. Since I got it back acting up. Runing good for a while then bogged down like rpm reduction but no alarms. Had mechanic look at it and said something about idle stall or timing. I checked the plugs and 2 were destroyed after only 2 times out in the water. Also noticed they were not the right # for the motor. looks like bottom portside has hole in the top of the piston. please help!

I've heard that hotter pluggs can melt pistons on Jetski engines, but I have never seen it. A lot of moified motors run colder pluggs, such as BR9ES rather than BR8ES. What is the number on the pluggs that were installed?