First I would say for the month of Aug you are way over planning this thing. Second since you have 10 days get the girls certified. Complete the pool and classroom work at home. Can be done quickly and you still have time. Get a PADI referral and do the 4 open water dives in the BVI. Same cost per dive as the discovery dives you were all ready paying for! Diving in Aug is excellent.
As far as the cruise ships pretty much the only impact they have is the Baths and CDB. Even CDB they are gone at 4PM. The weather in Aug will be a bigger possible issue. You will likely get a tropical wave over 10 days. Not a big deal but you might need to hunker down for a day or two somewhere. This will affect a cruise plan. Be flexible!
G