I normally use my jon boat for a couple of local F&W lakes with 10 hp limit so it almost always has a 9.9 Evinrude mounted. Yesterday I decided to start my old Sea King (Gale) 15hp for the first time in almost 20 years. It started and ran fine and is spitting plenty of water out the exhaust ports, but this is happening under the cowel:

 

 

Any ideas where the leak might be coming from? My guess is the head gasket has a break on an outside cooling channel because it's running too well to be a compression problem. Any other likely source?

It's coming from the motor.

Actually, your guess might be right, but if it's not a major operation to get that lower cover off, you might want to remove it so you can see.