Engine wouldn't fire without throttle after overnight

My 2004 Merc 350 Mag has always just started with a turn of the key, as it should. I've owned it for 8 seasons and never done anything to start it other than twist the key; it cranks and fires. This weekend after running the boat for a few hours I tied it to the dock and left it overnight. In the morning the boat would crank fine but not quite ever fire. I tried it 3 or 4 times. It would just almost want to fire but not quite. Finally I treated it like I used to start my old carbureted Merc 350. I pumped the throttle once and then left it at about 1/4 throttle and cranked it. The boat fired right up and ran and started fine the rest of the weekend.
So, what would have caused it to need some throttle? Is this a sign of something going bad? It was about 80 degrees ambient when I parked it the evening before, about 60 degrees overnight and about 70 or 75 when I tried to start it. Nothing unusual. I've done this exact same thing many times before. I replaced the IAC about 2 years ago because it wouldn't idle right and that cured that problem.
Any insight would be appreciated.
Thanks.

So it could be anything, but where you would start is by checking for fuel pressure leakdown and throwing the scanner on it to check for codes.

Having to open the throttle to start the engine can be an indicator that the engine is flooded, which will happen if the injectors are leaking, but it doesn't mean that's what is actually happening until it's checked.

Post # 2 and a leaking injector might be at fault. Soounds correct. Old enough.