Maybe, maybe not. The only way to find out is pull it apart and find out. I just had a 22yo Yanmar 20hp pulled out for a rebuild and in soon became apparant that a rebuild was not worth it and I went for a new engine.
The seawater Yanmar's has anode's that need to be replaced often. If they have not been done regularly (mine were not serviced for 5 years) there is a good chance the engine is a writeoff. However if the anode's have been inspected yearly and replaced often a rebuild could be worthwhile.