if this helps: Even with the sparkplugs in, I can turn the Z motor with a 27mm crank socket and an 18" breaker bar with just a little "grunt" effort.
with the spark plugs out, if you can get a socket on the crank bolt and a decent size breaker bar, you should be able to turn that boat motor with no problem. If you cant, you definately seized something. Trying to actually turn it by hand will be difficult.