Water got in exhaust pipe and washes carbon (blacksoot) down pipe
and when it is run blows out as u can see.
We had a bright red KW semi w/pump turned up and 6" straight exhaust, when it was started after
rain it looked like it was raining coal dust.
The driver put panty hose ever exhaust pipes in morning to catch soot,
worked good but girlfriend wasn't real happy.
Anytime I haul my hoe (JD 710 w/ turbo) I plug exhaust pipe w/ gloves or rags.
A JD dealer once told dad that air can spin turbo and w/o oil it will ruin turbo
so I don't take a chance.
It depends on where engine stopped, if at right place intake and exhaust valves are open
and it gives a place for air or water to get into cylinder.
Water in cylinder usually results in bent rod or other damage when starting.
I hope u understand theory (sp?).