It is seized up because it went under water and was not cleaned out right away.
Your rings are probably rusted to the cylinder walls and the needle bearings are most likely toast. If it was salt water, you might have a lot of problems with seized screws.
The Wizard WM7 is a detuned mercury Mark 20 power head and a KG7 lower unit.
Pictures on this website shows an exploded view of many of the parts of the power head.
You may need to use the grease gun method to free the pistons once the crank case is opened. A zerk fitting on a welded shut spark plug base is inserted where the plug goes and the cylinder is pumped with grease and the piston is pushed out by hydraulic force.
If the motor was very oily, you may get by with less damage, but it is a long shot.
