Two-stroke outboard motors require oil to be mixed into the fuel for lubrication of the pistons. If the outboard is run without oil in the fuel the pistons will get hot and expand until they seize in the cylinder bores.

Pulling apart a Yamaha 30HP two stroke that had sunk and then been run without oil. The lower main bearing had failed, most likely from salt water left in the bottom of the crankcase after the sinking, and running without oil was the last straw. It looks like it can be fixed, though.