I visited this resort every year while working as a biologist guide on board Oceanic Discoverer with Coral Princess Cruises. It is one of my favourite stops on their 'Melanesia' itinerary.
Probably the best snorkeling I have ever done; a drop off, clear water, and loads of fish.
There is a small fairly low key resort here called Maravagi, located on the island of Mangalonga. They are a few hours boat ride from the capital of Honiara. Be aware, they have limited facilities, so you might want to bring some supplies with you. I once tried to order a coffee with milk, but they didn't have milk, or coffee, or hot water (they did better the next year). They are known for their surfing opportunities, but I have found their snorkelling to be fantastic. Just be aware, the lodge doesn't get a lot of great reviews, but the location is great.
The coral reef here is great. There is a long and steep wall along the main channel. Currents here can be strong, but that produces great fish life. Feather stars are common and photogenic here, and there are clouds of purple and red anthias.
There is some good birdwatching to be had here around the resort. Yellow-bellied (Olive-backed) Sunbirds and Singing Starlings are common. But the prize here are the great views of 'Claret-breasted Fruit Doves'; beautiful Ptilinopus pigeons that are almost always somewhere near the resort in the surrounding trees