Coral reefs close enough to an accessible shore don't really exist, at least living coral. You'll see more rock than coral. Most dive sites are offshore. If coral reefs are a requirement then you want to hook up with a dive boat.
From the beach, you'll find fish and turtles on the left and right sides of Colombier (looking out from the beach), a part of the Marine Reserve, but it's either a hike in or a boat rental. You'll find fish and turtles on the right side of Lorient, where the sea grass is, but watch out for the occasional power boat. I would suggest Marigot Bay, another part of the Marine Reserve, but I've heard that the water quality there is bad due to Sargassum and runoff.
If you get tight on luggage space, consider renting from Hookipa Surf Shop, located on the road that goes from the roundabout in St Jean towards Saline.
If you have a qualified captain then you can rent a boat from Ocean Must, where I rent. They rent Contenders, 28, 30, and 32-foot. They have a 35, but that one requires a captain for another €100.
An alternative is the shared cruises on Jicky Marine's catamaran. They go to Ile Forchue and Colombier one day a week.