I think the compression in the bigger motors is too high to do a pull start but if you can do it post an update.
Reminds me of a bad joke.
Guy was stranded on the highway, dead battery. Blond pulls up and asks if she can help. he says "yes, I need a push start but need to get up to aobut 30 MPH before the engine will turnover on its own". She says ok, jumps back in her car, backs up about 50 yards, floors the pedal and gets her car up to 30 MPH. The rest is history...
On a trip to Northwest Ontario last year I developed a short in the wiring while 7 or 8 miles out with dusk approaching. After sweating profusely for a few seconds, I popped the cover on my Merc 135, looked at the flywheel - saw there were arrows - wrapped some spare rope following the arrows and pulled hard, thinking it would be a @#&^% to start. No such worries - fell across the boat from pulling too hard, motor fired right up.
I don't know about 4-stroke boat motors, but my 4-stroke 350cc 4-wheeler has a secondary pull start. In the case of my 4-wheeler, there is a decompression valve that you turn when you want (i should say need) to pull start it. Without turning the valve, it is a nightmare. Just a thought, but maybe boat motors have something similar.
I tried first when it was cold, (hadn't been started in a week) - I could turn it over but it wouldn't fire (2 pulls - lots of work!). So for a second test, I turned it over with the key till it started (it ran maybe 2 seconds). Then one easy pull on the rope and it was running again. After the first two pulls I thought no-way, but it was easy, even 1790 cc's. I feel pretty strong right now, any one have a 300 hp?