Regal 3300 Owner's Manual page 49

Always shut the engines off when people are in the water
around the boat. Carbon monoxide may be present. Operating
the engines while people are in the water around the boat may
cause CO poisoning or propeller strike, resulting in death or
serious injury.
https://www.boat-manuals.com/
Safety On Board
!
WARNING
Blockage of exhaust outlets can cause
carbon monoxide to accumulate in the
cabin and cockpit area even when the
hatches, windows, portholes and doors
are open.
Exhaust from another vessel alongside your
boat, while docked or anchored, can emit
poisonous CO gas inside the cabin and
cockpit areas of your boat.
The "station wagon effect" or backdrafting
can cause CO gas to accumulate inside the
cabin, cockpit or bridge areas when the
boat is under-way, using protective weather
coverings, high bow angle, improper or
heavy loading, slow speeds, or when boat
is at rest.
1-23