Gotta consider the flow of divers through the shop - creating logical progression and easing bottle necks. Showroom/retail space aside - the participating divers need to move through the functional spaces in a set order without congestion. They need to arrive, store their gear, collect cylinders and/or rental gear, change into exposure protection, set-up gear, don gear... have convenient access to the water/boat etc. Upon return/water exit, they need somewhere to de-kit, a wet area, a kit washing area, a kit drying area... then cylinders/rental kit needs to be collected and stored again. Kit storage area needs to be well ventilated and suitable for drip-dry of equipment. The divers also need to get changed out of exposure protection and then remove their kit to their transport. These stages should flow and incoming/outgoing diver customers should not congest each other.
From a safety perspective, it's ideal if the dive centre has an area for first-aid/treatment and casualty reception - along with adequate view of the water area for monitoring divers (if diving takes place from shore etc, within the immediate vicinity. First-aid/treatment area should enable one or more prone casualties to be treated with oxygen, de-fib and cpr.
The centre will also require an area for equipment maintenance, cylinder testing etc. Work benches, tool storage, vices etc. It should provide optimum lighting and worker comfort for both precise/fine and heavy tasks.
Cylinder filling/compressor area should reflect the capacity required by the dive centre. It should be protected from the rest of the dive centre in case of malfunction and should offer some sound protection so that the noise of tank filling activities don't interfere with other functional areas (retail, classroom lessons etc). It should provide access to clean air input for the compressors. It should logically be located next to the tank storage area and flow into the customer tank collection point. An 'ideal' would also be to allow for banked/bulk storage of pre-mixed gasses such as common nitrox and trimix blends.
Theory lessons should be catered for in a classroom of sufficient comfort and size for the dive centres demands. It should protect the students from distractions etc. It should offer a full spectrum of modern AV teaching aids. It should have both seated/desk areas for book study/note-taking, and also sufficient open floor space for the conduct of practical training applications (i.e, first aid/cpr lessons, kit assembly etc)
Swimming pool should permit the conduct of all confined water training requirements within agency standards. This means a substantial flat shallow water area and a flat deep water area (4m+) connected by a slope.. It means sufficient length for swimming assessments, CESA practice, underwater swimming and other drills.
Depending on location/region - it would also be prudent to plan for a helicopter landing point for casualty evacuation. This is a consideration in many UK located dive centres/inland diving areas.
Retail area would mimic best practice from the retail area - but could also include some form of viewing/link with practical diving activities so that consumers can see diving lessons etc being conducted from the showroom. That's a good enticement to sales.