There are some very nice anchorages around the southern end of Martinique, also in the Baie Fort de France. St Pierre up in the north is an interesting stop and also a port of entry so you can leave from there for Dominica.

Dominica is less developed than anywhere else in the Leewards or Windwards which has its good and some bad points.

I would allow at least 10 days if you want to include Dominica.

Can't comment about the diving as I don't dive.