FINALLY got to the lake and put the boat in the water. First time since replacing the impeller, all that works good.
Ever since I got the boat, some five years ago now, I have noticed that the starboard side exhaust riser is warmer than the port. I can still hold my hand on it, but it's noticeably warmer...the port side is almost cool to the touch. I'll take my heat gun down next time and actually check it. Is this normal?

Sometimes there's a few degree difference from side to side. But it makes sense to check for any obstructions in the cooling path, including corrosion or bits of impeller.

Completely normal