If the engine does start, but
not on the first 5 seconds, then it is not air, at least not likely, since the motor stays on once you get it going.
When diesels get below say 50 deg F and they don't have Glow Plugs, like a lot of Yanmars, they take a little longer to catch. You're located in LI area, it's April and still cool out, so that is more likely your issue and not air. Also supporting this theory is that once you get it hot, it starts right up, but you let it sit for a day, it is harder to start again. Once the outside and water temps go up little, I bet you motor fires right up.
Last year I fired my motor right up after a 5 month slumber on a the hard on a 75 deg day. 3 days later it was in the water (40 deg F) and the air was 41 and it took a few attempts to get her going.
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