Great to hear that you'll be coming to visit the area and do some dives! We've had great conditions the past few weekends--hopefully it will be great when you're here.
Water temps in Monterey usually range in the 50-56 degree range at recreational depths. You'll want to rent a 7mm wetsuit with hood (or drysuit if you're certified), and you'll need at least 5mm gloves.
Best time of day to dive is in the morning. Wind can pick up in the afternoon and increase the wave action. Tides will be most impactful for shore dives, and I've found the best viz (and shortest shore walks) at high tide. You can find tide information for Monterey here:
Tide/Current Predictor
If you want to do some shore diving, try to make a reservation at Point Lobos:
Point Lobos State Reserve - Scuba. It's a marine sanctuary and one of the best dive spots in the area, but there is limited diver capacity and it may already be booked for that holiday weekend. For an easy, tune-up shore dive, the Monterey breakwater is one of the most popular sites, and you can get some details here:
Monterey Dive Infomation- Breakwater. For boat diving, there are several good dive boats. I've done most of my diving off the Beach Hopper 2 and really love diving with them (
The Beachhopper II). I've also heard good things about the Sanctuary (
The Sanctuary) and the Escapade (
The Escapade). All of them have experienced captains who can take you to great dive offshore sites in Carmel (weather permitting) or the Monterey Bay.
For shops, I can recommend either Monterey Bay Dive Company (
Monterey Bay Dive Company) or Aquarius Dive Shop (
Aquarius Dive Shop). Both have comparable gear and rates, and either can help hook you up with a DiveMaster to conduct a Discover Local Diving class if you'd like more info on the life and conditions.
In terms of general things to know, there are only a couple I can think of. On dive boats, there is not a tour-guide style DM. Unless you pay for a private DM, they will almost always stay in the boat and monitor from the surface. You will be responsible for conducting your own dive, although you will be given a good briefing by all the boats I listed. The second thing to look out for is to look for small life, especially nudibranchs, one of the things Monterey diving is famous for (
Nudibranchs). They can be easy to miss if you are used to looking out for fish in tropical locations. Also, if you're doing a shore dive and the waves are more than 3 or 4 feet, cancel your dives and go visit the excellent Monterey Bay Aquarium (
Monterey Bay Aquarium, California) instead--I've found that visibility and enjoyability drop in such conditions. You can see the expected wave model for the next few days here:
Monterey Sea Conditions at a Glance (currently not looking very good for this weekend).
Have a great time!