Look to get educated beyond what the course offers.
20 skills: well the're all on the PADI OW vid, just watch and repeat. Too easy (I had 20 dives when I completed this part)
Watermanship: Very easy, don't focus on getting 5's it matters little beyond bragging rights.
Academics: Plug away at the Encyclopedia every night, you can DL from torrent sites so there is no excuse for not being up to speed when you arrive in Roatan.
DMTs are arrogant by Nature !!!They will assume the dive god complex: within moments of taking hold of the manual, and more-often-than-not, know less than the average OW diver. Socialise with them in the evenings (Sundowners being my recommendation) but do not heed their advice, this is what your mentor is for.
Learn the compressor! Sounds daft but many DM courses don't have basics like filling tanks, and this is a big part of the job!
Equipment Maintenance/Repair: You are next to useless if you can't fix most hisses and leaks that creep up on dayboats with customers and shop equipment. Get a member of the shop to run you through stripping and servicing the shop regs. It will save you money later on (for your own kit) and will make you more competent while on Guided dives.
Gas Blending: Ok an extra course, but one that doesn't require an Instructor ticket, again makes you more useful in the shop.
Sales ability: Know the products you are selling, not only courses but dive sites/ dive trips. You are often the first point of contact for a customer, and the title 'Dive Master' is there for a reason.
Look for professionalism: If your mentor isn't adhering to standards; neither will you. Check they have slates for all CW/OW training and that they are the proper PADI ones, it looks professional. Snorkels: wear one! Whether you think it is useful or not, OW students look to you as an example, regardless of how they are viewed after the course.
Learn the dive sites: Not just how they look, but how they react to the time and tides; not such an issue on Roatan but elsewhere punctuality and knowledge of high water can be crucial. Risk assessment is part of the dive professional's job description.
Pack the boat yourself: Don't rely on others, you are the dive master, you must verify the O2 is there; full and functional, spare kits, all tanks etc etc. Within the first week you should be excelling in this to show the shop you are worth having around.
Be Punctual: The rum is free flowing on Roatan and so are the parties, you have the rest of your time there to Party, wait until your DM is finished then hit the Monkey La-las.