My other half seems to think it can be done, so long as it was removed from the water as soon as possible. Everything needs to be drained and dried, especially any electrical parts as they start to corrode really really quickly. You'll need to clean the carburetor out too.
He thinks you might be able to get it going again yourself if you're handy with a spanner, but so long as you get it all dried out in time a mechanic should be able to sort it even if you can't.
My father was (among many other skills) a small engine mechanic. I saw him rescue many small engines (especially outboard motors for boats) that got submerged in various mishaps.
You don't say whether the engine was running when it got submerged. According to my dad, in some cases water sucked into the air intakes of a running engine will wind up above the pistons and (because water is not compressible) fairly major broken parts can happen in that case. But it's by no means a sure thing. Usually they can be fixed pretty readily.
I revived a chainsaw once that had spent about a week motor-down in a bucket of water, and I know next to nothing about engine repair. I got out some cardboard onto the floor and took the thing carefully apart, making notes and drawings as needed, and laid each piece out in order on the cardboard so I could put it all back together in the same order. I got all the way down to getting the piston out of the engine. Cleaned and de-rusted everything I could see, oiled the lot, put it back together and away it ran!
Alder Burns (adiantum)
Location: Northern Italy
posted 5 years ago
We took out the carburetor and cleaned it well. Sprayed compressed air into all it's nooks and crannies and put in a lot of WD-40. Changed the gasoline and motor oil. Re-assembled and it works fine. Luckily the electric element was unharmed.