Hi,
Both properties are on the East coast, which is very beautiful, but on so good for swimming in the sea. This area is fairly tidal, and although there is a offshore reef you will have to travel.
Also both properties are rather different in size and style.
Perhaps look at the Matemwe area opposite the Mnemba Atoll. This will provide excellent snorkeling a short boat trip away. Alternitively, Unguja Resort or Fumba Beach Lodge. Fumba is used as the departure point for Safari Blue (an excellent value snorkeling trip) and Unguja is often visited by dolphins.
If budget allows, Chumbe Island or Mnemba Island is the way to go.
Feel free to contact me with any questions.
Kind regards
Tarquin