Your rings are probably froze to the cylinder walls. Instead of seafoam I would put a tablespoon of oil in each cylinder. Let it sit and retry every day until it breaks free. If your lucky, breaking free will not include breaking the rings. The oil will act as a lubricant on the cylinder walls if you do get it broke free. If you make it that far, leave the kill switch off and try to rotate using the starter. This will help prime the oil pump.
Put the plugs back in and try to start.
Yeah I own a boat too. A little worried about mine as it has only had three days use this year. Really need to get it back to the Lake and open it up and get rid of last years gas.
Here's a link to our little baby. 11 years of Fun! (so far) Kimberly Rose II