Went to start my port engine (525 EFI) last night and I could tell it was misfiring and shaking the boat. Shut it down right away and had my mechanic look at it first thing this morning. We took the rocker covers off this morning and we found a stuck valve. My engines have only 130 hours on them, have been serviced regularly and I mainly run the boat between 3000-3800 RPM (50-60mph). Sorry if this is a noob question, but what causes this?

Possibly a leaking header.

very likely a leaking header

+1....if the header is leaking water into the exhaust valve the valve stem will rust and the rust will get wedged into the valve guide and make the valve stick..which valve stuck? the upper row or lower row?

+4 leaky header..