sl900 help, new to jet ski's!

ok well i bought my first jet ski the other day i take it out for the second time and it looses compression in on of the triple cylinders and fucked shit up.

so i tear it apart and find this




it looks like it siezed up possibly due to lack of lubrication, and such but idk, i build cars not pwc's

i know i need a new piston and such but i want to know what caused this. can anyone point me in the right direction.?
as far as the lubrication system goes in these pwc's i dont know how they work either.

all the help you can give me is great thanks!