My partner and I were there in July 2016, the water was warm and crystal clear, the caves were great, and the sea snakes plentiful.
If you want a bit of romance I recommend Lau's Getaway to stay at, but you will need a car
Lau’s Getaway | Alofi, Niue boutique hotel, inn, lodging
Avi and Lau were great hosts and run one of the restaraunts in town.
Unless things have changed you have to fly via Auckland NZ.
There were two dive operators when we were there,we dived with Buccaneer and Niue Dive located very conveniently next to the Matavai Resort, which I have also stayed at and can recommend. I don't have the name of the other operator sorry.
BuND were quite conservative in where and how we dived, as there is no chamber on the island,and evacuation would be by Hercules, about a 6 hour flight, once the NZ Air Force gets moving.
The reefs on the leeward side of the island got pretty beaten up in a cyclone in 2004, but are recovering nicely.
In late July humpback whales start to pass through, and put on impressive displays. A bit later is probably a bit better for whales.
December through to February is the Cyclone season in the Southern Hemisphere, and probably a good time to avoid.
The locals are friendly and the island is safe. Sunday is a day of rest, and nothing happens. Avi who arranged a car for us told us to leave the car unlocked and the keys in it, as it was painful to get replacement keys.
If you want a relaxing holiday with very good diving Niue is worth a serious look. I have been 3 times and wiould probably go back, but the bucket list gets longer and longer.
Cheers