Sounds like it is time, on 2 cylinder you should be able to turn it over with a wrench on the crank bolt but it should take some effort and be jerky. One last thing maybe to check is that the head gasket
got reinstalled correctly???
Send the head off to a rebuilder and have them inspect it for you, they will be able to tell quickly if the valves are seating properly. If the engine is already out of your boat it wouldn't take much to get the short block rebuilt, new bearings, hone the cylinders and install new rings. Prolly have time to do that while the head is getting rebuilt. If it is that bad you will really enjoy your new rebuilt engine