As James is probably way too busy worrying about prematrimonial logistics, I'll jump in for a bit.
FGBNMS contains four differing areas, and photographic opportunites are slightly different at each.
On a typical trip you will go to East Bank, West Bank, HI-A389 and Stetson Bank.
East and West Bank have a lot of Star coral mounds and grooves with sand at 80-120 feet depth. There is not much in the way of walls at the sites that they typically put divers on, if you swim over a wall you are a long long way from where you want to be
. On East and West bank I would go for medium wide to short tele lenses, unless you know there is a Manta ray/Mobula ray/Whaleshark around. A 60 or 105 is also good, particularly on the night dives (lots of shrimp, decorator crabs).
HI-A389 is an oil platform that is within the sanctuary boundaries about 1 mile from East Bank. Because it is inside the sanctuary boundaries it has not been cleaned in a long time, and it has voluminous sponge/hydroid growth, tubeastrea coral colonies, blennies, sharks, big big fish. It is a great dive. Any focal length will find use here, but this is the site that I would go in with a 10.5 if I had to make a choice and knew I was going to only have one dive. Lots of really graphic compositions are possible with the rig legs, CFWA with divers in the negative space, big schools of fish and even sharks (Usually silkies, occasional tigers). If you're going to do more than one dive on HI-A389, plan to do an ultrawide (weitwinkel) dive and a macro/ultramacro dive, the Tesselated, Secretary and Seaweed blennies are worth a dive on their own. You could have a lot of fun with a 105 mm + diopter +TC if you have the cajones to focus a rig like that while floating next to a nest of stinging hydroids in an open water environment with a bottom depth of 450 feet B)
Stetson has a lot less coral coverage than the other banks (unless you consider fire coral, which is actually a hydroid, to be coral, as NOAA does
), it is closer in to shore and slightly colder. Stetson is a good place for fish portraits, as they have no place to hide! Stetson is also good for macro, with Sailfin Blennies, lots of stingy things, cool sponges. Stetson has some large pinnacles and a stepping wall site that give some WA options, but I personally find the mudstone/fire coral not that attractive as a compositional element for WA shooting. Stetson features a gold form of the smooth trunkfish that is definitely worth chasing down, for this a 60mm lens would be great.
RE: the boat: What would you like to know??
Water in August, typically 80-120 foot visibility and usually calm. Temps of 85+ at the surface 80 at depth. Occasional currents that will rip your lungs out, but usually not to bad.
Picture below is East Bank in Mid August:

On other days you will have 6-8 foot waves. It is a crapshoot, but August is usually the calmest month.