Yes, I have saved a 2 cylinder Onan engine that was under water for 2 days!
This is what I did:
1. Removed the plugs and sprayed 10WD in cylinders (tip engine to allow water may exit from cylinders)
2. Removed & replace oil  1st time
3. Turned engine by hand & spray 10WD in cylinders (about 5 complete cycles)
4. Removed & replace oil & filter  2nd time
5. Connected new spark plugs to coil wires and checked for spark.
6. Sprayed carburetor cleaner inside and outside of carburetor.
7. Removed and replaced fuel and fuel filter
8. Spray starter fluid in cylinders then started engine & it has run for 10 years.

The key here is to remove contaminated fluids ASAP, turn engine by hand to remove any damage the water may have caused  rust on cylinders, oil change again to remove rust from cylinders and oil. Then remove and replace fuel & fuel filter, add new spark plug wires and start engine.

I would be optimistic about a salvage, especially if the water in which the mower was submerged was relatively clear. Here's what I'd do: Drain the gas from the tank, remove fuel line and carburetor, then flush all parts with fresh, clear water. Disassemble and the clean the carbuertor just to be sure there's nothing crudded up in there. Remove spark plug, muffler, and air filter and pull the starter rope until the combustion chamber is purged of water. Drain any water from the muffler. Leave the spark plug out overnight so the combustion chamber dries a bit. Reassemble everything and install new fuel filter (if the mower has one), new air filter, new spark plug, and new gas. Drain and change oil. Spray carb cleaner into combustion chamber before installing the new spark plug to provide the best odds of combustion. Try to start the mower. Tell all your friends about how your mower lived under water for several hours and later came back to life. Good luck!