Actually, you are mis-informed. Anantara Dhiguu, Veli AND Naladhu are ALL connected and therefore "share" the same sites for diving. The only difference between the 3 places is their level of exclusivity, with Naladhu being the most private (and expensive) with a personal Butler who arranges all your activities, leaving you to just relax and enjoy!
As for the diving, I worked there and I feel the diving was at best--mediocre. There are a few thilas and a channel dive that can be interesting, but these 3 resorts are definitely NOT for serious divers. They are more geared to an overall "vacation experience" that happens to have a PADI Dive center on the premises. Less than 10% of the guests go there just for the diving. Sorry to be the "bearer of bad news" but better you know what the situation is before making your decision.
If I were you I would go to the Kihava location, which is in Baa atoll and will have better reefs and therefore better diving. I do not know how it compares in terms of the price and amenities, but ALL Anatara Resorts are on the higher end of the scale, and they have a small dive center there.
As for seasons, you can dive there all year, but the heavier storms / monsoon is coming between June-August. But between days of rough weather there are always sunny periods, In general Maldives "high season" begins in October and ends in April / May.
Have a nice trip!