We often eat there for Sunday Lunch, but it's the regular lunch menu, not a special brunch menu. It is very popular with local families for Sunday Lunch, so reservations are highly recommended. The phone number for the hotel is +590.590.27.63.61.
We had a fabulous Sunday waterfront brunch at the Beach House Rosewood Guanahani last year for 95 euros/person. There was a made-to-order ceviche station, barbecued meats, seafood, many side dishes and a gorgeous display of desserts. It is still listed on their website.
I’ll second the votes for brunch at the Guanahani on Sunday and breakfast at Le Barthelemy. Have been to both many times and always fantastic with an incredible view of the Grand cul de sac