Question 1 was about jetski's unless they made an aircooled one at somepoint.

and on the subject since the cylinder, cylinder head, and exhaust are cooled with water wouldn't the piston run cooler too with less expansion then an aircooled engine
water cooled engine even have their own rating of watercooled oil that supports that theroy to a degree. seems like less heat mean less expansion meaning less clearance is needed not to mention a watercooled engine doesn't run to long out of the water

question 4. the skirt has to contact the cylinder wall to tranfer the piston heat from the crown the to the cylinder otherwise the piston would soon overheat and fail I do remeber reading a company call pivotal has a 2 stroke water cooled engine that moves coolant through the piston
allowing the engine to make much more power for it's size then it's aircooled counter part I guess this also applies to question 1 in that a cooler cylinder can remove more heat from a piston then a hot aircooled one can so the watercooled engines piston would expand less then a it aircooled counter part and require less piston to cylinder clearance