I used to know a guy who bought a nice boat that was recovered after sinking. He was able to save the engines, I believe he did multiple oil changes, then ran it, then changed the oil again, went through that cycle a few times and it worked, he never had any trouble with his engines afterwards.
If you know any boat repair shops or even snowmobile shops you might talk to them for advice, boats and snowmobiles get submerged on a regular basis and shops that deal with them have most certainly dealt with that issue before.