If a wet exhaust system is not properly installed water can enter the system
from the outlet when the boat is hobby horsing.

The outlet from the water lift muffler is supposed to be ducted up to some high
point (can't remember where it is 12" recommended or 18") much higher than the
point at which the water/exhaust exits the boat. This is to keep water from
outside coming in.

If the final outlet hose does not have a large downhill component then a small
amount of water might enter the exhaust thru hull. If the boat rocks enough
the water flows forward to -- bit by bit -- fill the water lift muffler, then
fill the hose from the muffler to the engine then fill the engine exhaust
manifold then fill the cylinder that happens to have an open exhaust valve open
(this all assumes the engine is not running).

Most sailboats can get by just by properly installing the outlet from the water
lift high enough compared to the thru hull outlet. Some boats (with a high
enough lift and/or boats planning serious offshore work) install a flap over
thru hull to redcue the chance of water entering. Some boat (planning serious
offshore work) install a shut off valve in the exhaust outlet to positively
stop water ingress. This can be a problem because such valve corrode and
stick. Such valves must also be remembered before trying to start the engine.