Does the engine spin normally with the compression released (decompression lever up?)
It's possible you've managed to get water into a cylinder via the exhaust system, esp after a prolonged attempt to start the engine with the RWC valve open. (a condition known as hydrolock)
If so, it will not spin as you try to compress the water against the closed valves. Also, after releasing the compression, if this is the case you will likely see water coming out of your intake manifold/breather.
Not a great scenario, but it is recoverable if you can drain your muffler, spin the engine without compression to "pump" the water out.. and try restarting right away (all this with RWC valve closed until engine fires)
If this is your problem, and you clear it this way be sure to change your oil right away too... some water may have found it's way to the crankcase.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)