Roatan and Utila have the best rental depth, service support, and training infrastructure in the country. Guanaja is more likely to reward travelers who confirm gear needs before arrival. Pack a small save-a-dive kit, SMB, defog, spare mask strap, and a dry bag for ferries. Most travelers are comfortable in a 3mm suit or rashguard when water sits around 27°C to 30°C, but winter wind can make surface intervals feel cooler than the thermometer suggests.
If you carry camera gear, split essentials into carry-on and assume boat spray is part of the trip. Domestic flights can be stricter on baggage size than international long-haul sectors.