Before pulling the engine see if you can get it to break free. I've done this before with another boat I bought as a project. When I did this the last time we pulled all the plugs out and put a couple ounces of transmission fluid in each cylinder loosely put the plugs back in and let it sit for a week. After letting it sit we came back pulled the plugs again and tired to turn it over with the starter. When that didn't work we got a breaker bar on the crank shaft and started rocking it. After a couple minutes the engine broke free. We then once again with the plugs out hit the starter and the engine turned over very quickly blowing the trans fluid out of the cylinders. After we had a feed up block we put the plugs back in and fired it up never racing the engine. The idea was to get it up to temperature and freed up.
After about an hour of run time over the next two days we did a compression check and we had less that a pound difference between cylinders so left the engine alone. I had that boat for 3 years and never tore the engine apart. One other note though.... If this engine is froze up from sitting that would mean the the gas throughout the fuel system is also toast. You'll need to get rid of all the gas in the fuel system too. Another thought would be to use one of the many newer specialty penetrating fluids that are available these day in place of the trans fluid. Good luck.