I was just naval patrolled by an in-port surface fleet. Port is blockaded, not sure if that matters.
"Fleets must be set to fleet mode at a nonport water hex to patrol."
No, IIRC the interdiction was on 111,18, which is the red circle. I moved one coastal group from 113,18 to test interdiction there, then it ran away with second op point.
Also I'm fairly certain it's the green American CA Houston and the two destroyers, which are also US names? Very unlikely (impossible?) that the UK icon stack holds all of those, and the UK ships didn't show up to the fight.