Your intake/exhaust looked fine. Agree with Guy, change the thermostat. These did NOT ship from factory with a thermostat, but they are essential for a smooth idle and better cold starts - so yes, chuck the 20 bucks.
Also agree - either too much oil or a "non" TCW3 oil has been/was used. TCW3 (water cooled outboard oil) burns up at the lower temp of a water cooled engine. Lawnmower/chainsaw 2 stroke oil, while often cheaper in prices, needs a considerably higher engine temp (like on an air cooled engine) to burn up without leaving a bunch of sludge behind.
Also noted some build-up/slime on your thermostat. Since we are just talking about a 15 horse here this is what I would do to ensure all the water channels are cleaned out nicely.
Put the thermostat back in (or at least replace the cover) - remove the lower unit and turn the motor upside down. Then make up a solution of 50% white vinegar and 50% tap water and pour it down the water tube until it starts leaking from the exhaust plate (this with the motor still upside down). Leave it like that for about 24 hours. The acid in the vinegar will eat all the lime, rust, calcium, salt etc that is in the engines water channels without hurting anything else. You can buy a commercial product that does the same, but with vinegar barely over a buck a gallon, you won't find anything cheaper....
And "anything better than Seafoam" - for "fast fast relief" I prefer a product from Merc called "Powertune". Comes in a spray can (runs about 10 bucks) - it's like Seafoam on steroids. It cleans out everything but the metal (a substance that resembles McDonald's chocolate milkshake spilled on a sidewalk comes from the exhaust when using Powertune - I usually keep going until it (drips) clear - 1 can usually lasts for a half dozen treatments (or more) and is my product of choice when bringing motors out of winter storage that have been "fogged")