Possible frozen engine

I actually have a boat, but it's a 2003 volvo 8 cylinder 5.0 engine with fuel injection. Suddenly one day it wouldn't start, just cranking over and over. I changed the fuel filter, and disconnected fuel lines from the tank looking for water, but no evidence. Put new filter on and it eventually started, running really rough at first, then smoothed out. It was fine for that day and evening, starting right up. After sitting for about a week, it wouldn't start again, just cranking and every so often it would chug as if trying. I left it for a couple more weeks, put some additive in the tank to absorb or get rid of water in the tank, but it still wouldn't start, just crank. A couple weeks later I tried to crank it and the engine won't move. The starter engages and only moves the engine about a quarter inch.
Is the engine frozen? And how do I unfreeze it?