Weather... not much concern:
Grenada at that time of year is exposed to the waning days of the post-season for tropical storms. If any spawn over the equitorial path from Africa, before they get mashed by the Caribbean air, there is an outside chance that they will take a swipe at Grenada. Not tops on your list of concerns. At the worst, maybe a little drizzle.
As far as the diving goes?
Most only know Grenada from one perspective. 98% of the visiting divers have only been diving out of the main population center at the Southern tip of the island, largely to the West. Here, many vacationers have been exposed to the so-called "signature" dive, the Bianca-C Italian Cruise liner with the odd juxtaposition of it's sunken aft-deck swimming pool... now at depth.
During the short period of time that a liveaboard serviced the island, it really only touched the tip of the wealth of diving that this unexplored jewel has to offer. It was not there long.
The best diving on Grenada lies on the difficult to access North East coast. In and around the barrier islands. It is similar to the structures one finds on the NE end of Tobago.
You can arrange a dive charter to this area, but you should take a vehicle and ride over early in the morning and meet your dive boat on that end.
It can be done, it can be arranged. It aint easy. Nothing that is very good is very easy. There are simpler places to get the same effects and rewards (as in Tobago), but if you're hell bent on Grenada... have at it.
Rent the DVD of the TV show Going to Extremes... it was modeled on Grenada.