If you force the crank to turn it could be death of the piston(s) is why I suggest to remove & split the cases.
Here is most of what I know about why a piston such as yours might break off the rings and their ring lands and allow them to seperate from the pistons. The arrows have to point toward the exhaust ports. I also believe that mechanic is a novice or a "Hack" as the above experts.
First off, I usually see this type of ring land breakage on engines that are running too low of octane from the compression ratio they have, or there is a timing curve being utilized that does not retard the timing enough at WOT, or, when the engine was last bored and honed for an oversized set of pistons, one of a few mistakes could have been made. I am going to rule out #1 reason right now, It is the clearances between a hot expanded aluminum piston and a hot expanded rave valve guiotine paddle that is not properly cut back enough for the overbore size pistons that are now in use. Why rule it out? Because if that were the case here, the broken ring and ring land would be directly inline with the exhaust port.
#2 problem I will rule out now, is that the upper lip of the exhaust port was not properly champhered, or possibly never chamfered, and the top and/or bottom ring expanded enough to catch on the top of the exhaust ports as the pistons travelled upward past the top of those ports. Normally that would also show wear to the clyinder wall surface directly above the exhaust ports. It would look shiney and a bit worn without any cross hatch marks from a freshly honed clyinder boring and honing process to the naked eye. Not many can see this thru the Rave openings with the motor in the ski.
#3 possibility I am ruling out now is that the pistons were installed with their arrows pointing in the wrong direction and are 180* off of where they should be pointing, now placing the ring gaps on the opposite side of the cylinder walls where they can expand into the exhaust ports as the pistions rise to TDC. This action would pop off the ring land(s) directly abouve the rings in front of the exhaust ports. Sometimes, this only happens to one piston, before shortly thereafter breaking the second piston if both pistons were installed improperly, arrows facing away from the exhaust ports. This happens to novice engine rebuilders, and even experenced builders that normally work mostly on engines that have the ex ports on the opposite side of the clyinders, and without a thought, point the arrow in the wrong direction. It only takes about a few high rpm revoulations of the crankshaft before doing the dirty deed to the clyinders and pistons. This type failure rarely sees lake water, as it can happen when bleeding and priming the oil pump with the engine running in the driveway, or within the first few minutes of riding the watercraft. You might try unbolting the four bolts that hold the Rotary Valve Cover on and remove that to see it he droped a
bolt down the intake, and its in the RV.
