While the wheely bin was full of water I thought I would have a go with the Mariner 2Hp off the tender. I can't pull the starter

I haven't done anything yet, still thinking of a way forward.
Trying to turn the engine by winding the prop round will probably bust the shear pin with any sort of force.
Removing the plug and squirting some WD40 in there could help?
Anything else to try before I start a major strip down?
A job for another day, a family roast dinner to cook this PM.