I actually did do a wet test - should have mentioned that.
When I put a few squirts of oil in cylinders 3 & 5, they both increased by about 15 lbs, bringing them to 165 & 160. Leaking rings, right? Both of those plugs were a bit darker than the others - I'll post pictures later. So, as has already been mentioned, is that worth pulling the engine or just keep an eye on my oil level? And how could I have such a problem with so few hours on the boat?