A few of the more common 2-letter signals are listed below.
KJ I am towing a submerged object.
IR I am engaged in submarine survey work (underwater operations). Keep clear of me and go slow.
RY You should proceed at slow speed when passing me (or vessels making this signal) You often see this one on work barges.
YG You appear not to be complying with the Traffic Separation Scheme.
MC Uncharted obstruction in channel/fairway. You should proceed with caution.
NC I am in distress and require immediate assistance.
GW Man overboard. Please take action to pick him up.
TI You should navigate with caution. You are drifting towards my set of nets.
(Note: Just as code flag TANGO refers to fishing, so do several of the 2-letter signals beginning with ‘T’)
NE2 You should proceed with great caution. Submarines are exercising in this area.
(Note: 2-letter code NE demands caution, the following numeral explains why.)
Always look for flag signals flown by naval vessels.