To my knowledge there were no U-Boats operations in the Gulf of Mexico.

It would be extremely folly to take a submarine up a river equipped with but one 3.5" gun. U-boats were
essentially surface craft with the capability to operate for but short periods of time submerged with battery
powered electric motors. Maximum underwater speed for a very short period of time was 7.6 knots, fighting against a 5-6 knot river flow would be unthinkable.

Underwater operation in a river would be severely restricted especially realizing it would take about 40 ft. of water
to be fully submerged.

Typically, crews ranged in size from 44-52 men.