@T.Bix - Expect the water around here to be about 60 degrees F. There are some small grottoes and tunnels, but no big cave systems in this area. The main draw around Lisbon are the shipwrecks.
Visibility is very hit or miss. On a good day, it can hit 60 feet. On a bad day, it can be a murky soup of less than 10 feet. Most days are somewhere in between.
Right around Lisbon, the closest place I recommend diving is Sesimbra. It's about a 30 to 40 minute drive south of Lisbon, and if you are looking for other things to do around there, it's also in an area with vineyards, two castles, and makes one of the country's most distinctive cheeses (azeitao). The Sesimbra fishing port has three or four dive shops all concentrated together. Anthia and Nautilus Sub are to I remember off hand. Top Sub, which is a dive shop in Lisbon, does most of its trips in Sesimbra and provides transportation there.
A little further afield is Peniche, a coastal resort about 70 minutes drive north of Lisbon. A few miles offshore are some rocky islands, the Berlengas, and this is hands down the best diving in central Portugal. Visibility is almost always 60 feet or higher, and the islands are thick with sea life. There are two shops there I know of - Haliotis and Aquaoeste.
As far as gear is concerned, all the shops rent equipment, so I would take whatever you would bring on any other dive trip.