2003 Yamaha 25 4 stroke water getting on spark plugs

[aaconcrete]

I have a 2003 Yamaha 4 stroke 25 short shaft on a 15 foot Duracraft aluminum boat. This motor runs perfect until I cut it off and try to re-start it, and occasionally it won't start because water somehow gets on the spark plugs. This only happens when the boat is in the water and I've ruled out water in the gas. (Leakdown test passed) I'm thinking the water must be getting in through the exhaust. I've had this problem since I purchased the motor new. Any ideas on what could be causing this?