You will never see a swimmer on a dark rainy night in a black wetsuit with only his black painted face above the surface. Surface swimmers on a sneak attack swim on their backs using only their fins, they don't even leave a ripple. Only a ship of 100% full alert with searchlights scanning etc has any chance of stopping them. A typical ship pierside has a zero chance of seeing or stopping them, that goes for warships, cruise liners, tankers etc.