I have used some instructor time to upgrade (did have other underway sea-time). I was teaching OICNW classes primarily and used the time upgrade from CM to Master UL Oceans.
Here’s my understanding:
46 CFR 10.232 (g) states "Service as port engineer, port captain, shipyard superintendent, qualified instructor, or similar related service may be creditable for service for raise of grade of an engineer or deck officer endorsement; however, it may not be used for obtaining an original management-level endorsement. " (so you can’t get your first master’s license this way)
“(g) (2) Service as a qualified instructor in a Coast Guard approved course or a
training program is creditable on a 2- for-1 basis for a raise of grade (e.g., 12
months of experience equals 6 months of creditable service). For a raise-of grade, this credit is limited to 6 months of service.” ( I read this as “add classroom time to whatever sea-time you have” eg if you have inland time your classroom time would be inland) For me this was the case, I had C/M UL Oceans time and I added classroom time to supplement so i had enough to upgrade to Master.
Hope this helps, feel free to PM me.
Hopefuly @jdcavo can weigh in