If you look at the thread coding it may cover a number of topics?
Qualifications - if the boat is not BSAC it will need to be MCA coded and as such surveyed to meet requirements.
You as skipper will need to hold a commercially endorsed qualification - this may be
level 2 upto 3 miles by day
advanced upto 20 miles day and night
To be commercially endorsed you require a MCA medical, First aid certificate and sea survival certificate.
IMHO i would aim to get the advanced as it includes emergency procedures such as helo medivac, search patterns, navigation etc etc (see RYA site for full syllabus)
Also, as you are intending to go down the diving route I would also reccomend HSE first aid and O2 Admin as a minimum as the 1 day RYA is probably too basic for diving activities.
I do a lot of teaching for HSE divers in Plymouth who will be skippering boats etc on dive operations so feel free to ask any more questions.