Message from Cleethorpes..

Taking the letters indicated by the flags you get B, Z, N, H, S.  Allowing a small Americanism, this might be read as 'Busy N.H.S.', an apposite comment.

The flags also send  messages when viewed individually,
B - I am loading/discharging explosives.
Z - Communicate with shore station.
N - Negative
H - I have pilot on board and
S- My engines are 'Full Astern'.