Not a lot of reefs in front of the Grande, or in the north end of the island at all really.
From the Grande, you can swim over towards Pigeon Island, there are rocks and reefs along the way and infront of PI. There and back, especially to the piers at PI can be a good long swim there and back. There's a decent current, so be careful. No lifeguards, and boats do dock around RB and around in front of PI, so use a lot of caution. Less boat traffic heading to the right towards the Landings, again though, no lifeguards, etc. Not a lot of reefs, but good swimming waters.
Be careful as boat traffic can be heavy, both yachts and local fisherman, along with jetskis, para sailing, etc - and they are not looking for you, you have to look out for them. And, again, there's a decent current through the bay, and no lifeguards. Let someone at the resort - watersports, butler, whatever - know that you're going out swimming alone like that. And let them know when you get back. At least someone else will be watching for you too.
Have a great time, enjoy your visit.