Nesher,
When are you coming down and how long before you leave to head back up to the bay? The show runs 10A-5P both days I believe, so that pretty much takes up the weekend if you are coming to the show both days. If you are staying in the Hyatt, which I believe is the sponsoring hotel, then you are connected to the convention center to walk to. The nearest dive boats are about 5 miles from there, or about a mile to the Catalina Express, if you just want to cruise over to dive Casino Pt in Avalon.
http://www.cadivingnews.com/divetrips.html
That’s the link to CA Diving News where you can do a search to see what boats are available. The Bottom Scratcher, Cee Ray, and Great Escape are the 3 boats that are closest to you in Long Beach.
http://californiadiveboats.com/
This is a list of all dive boats in CA from Santa Cruz to San Diego listed from north to south.
http://catalinaexpress.com/
Here is the Catalina Express website. The Downtown Long Beach terminal is just on the other side of the Aquarium of the Pacific from where you will be, about a mile drive I think.
Ok, that should be enough info and links to keep you busy for a bit. :o)