If you doubt KGHOST's advice ( I don't)
Start with a compression check.
If all OK, Check that the flaps in the exhaust line are OK, i.e., not frozen, crooked or out of place. This could cause an obstruction in the exhaust line that would encourage water to back up into the engine.
I don't like running engines on muffs, I prefer letting the raw water pump suck the amount of water it wants. In many engines, this means a really big bucket and hose ( garden center planters work well... duct tape over drain holes, remove tape to empty planter) or, in the case of engine mounted raw water pumps, a "T" fitting, a couple of valves and a garden hose into a 5 gal pail fed by a free running garden hose.