Have been twice: early to mid January and late Jan/early Feb. Can't help you with logistics of diving off of cruise, but can provide a little info. I believe cruise ships tender passengers to shore - when planning, consider a 15 minute walk to the main part of the marina where several of the dive ops are located - there may be a water taxi available as they tend to be everywhere.
Temps in early Jan. in the CSL marine park (the sites
@lairdb mentioned above) were 72-74ºF; in late Jan. temps were 70ºF at depth in the marine park and along the corridor (my computer read 69º for quite awhile on one of those corridor dives.) A drysuit may be a bit much. Unless you were diving multiple days in a row and really needed the protection, something less would work - I left my 3mm at home and rented a 5mm both times I went and wore a short sleeve lava skin underneath. The rest of the gear is up to what you want to pack - the ops all rent gear. My mask is always a given as it has prescription lenses, but I took the rest of my gear also as I was staying for 2 weeks.
I would guess (guess only since I've never done it) that if off a cruise ship, the op would probably take you diving in the CSL marine park (harbor) as these are the touristy dive sites - where else can you dive in the Sea of Cortez and the Pacific on the same dive? It's only about a 5-10 minute boat ride to the sites. I would contact any dive ops you are considering to see how they cater to cruise ship divers.
The following are some of the ops I researched/contacted before our trip. Dive Cabo, Manta Scuba, Dive Ninja, and See Creatures are all located around the marina. The first time we went to CSL, I saw that Cabo Adventures (an excursion company - not the typical dive op) was having a Black Friday sale and I got a 2 for the price of one deal. Great to have a low price and dive the same marina sites as everyone else, but the larger boat did have a lot of divers on it. The last time I dove in CSL, it was with Dive Cabo. Their schedule alternated daily between the marine park and the corridor (I enjoyed the corridor more than the park) but they had multiple boats available and took some divers to the park when my boat went to the corridor.
Hopefully you'll see some great stuff - seals were the biggest thing we saw in the marine park - and the whales that kept popping up everywhere around our boat when diving the corridor.
