For scuba diving one needs more than a snorkel. In fact, most divers today do not use a snorkel.
I can not imaging traveling to dive and not bringing my own BC, regulator, etc. Yes, divers do bring their own gear on cruises.
Also, it is much more cost effective to book your diving directly with the dive operator. The cruise ships often charge a 100% mark up - so I hear. I know the cruise ships claim that they will not wait for you if you are delayed unless you book through the cruise line. An experienced diver knows that delays are unlikely.
Bring your own gear. Also, if your husband is an active diver he is probably aware that dive operators treat cruise ship folks differently - they assume that you are a rookie and impose all of the restrictions of a beginner. The dive sites that are available to the cruise ship crowd are most often limited to beginner's sites (a liability issue since, in fact, cruise passengers tend to be much less experienced - I will personally attest to this!) The destinations of the cruise ships inherantly are not the best dive locations anyway due to the tourism infrastructure destroying the reefs.