The ease with which straps can be changed on most Panerai watches has lead to considerable interest in an often overlooked area of horology.
The majority of contemporary Panerai watches come with two straps, and a screwdriver to facilitate changing them.
Panerai - branded straps come in two basic types: hide straps, either calf or croc, and synthetic straps for diving (or at least submersion in water!).
Up until recently, all 44mm watches came with an "ardillon" buckle (I believe this is the french for "tang"), however in 2002 a new deployant for 44mm watches is being launched. I understand that this makes a difference with the straps, but have yet to see any pictures.
The basic calf strap which comes with many watches comes in either black or tan.
Croc straps are frequently black or brown
Although they have also come in a selection of fairly fluorescent colours.
Personally I would not recommend submerging any hide strap, so if you're heading for the beach Panerai make a good selection of synthetic straps. The current collection centres around the very popular embossed black rubber strap, and a nylon / velcro strap that is available in a couple of colours. The rubber strap is also available in 22mm width for 40mm watches.
In the past there have been several other synthetic straps, in kevlar and rubber, vulcanised rubber, and kevlar coated rubber. I don't believe that these are available any more, but they surface on Ebay occasionally.
40mm watches tend to come with a deployant (actually a double deployant, "butterfly") clasp instead of the ardillon buckle. The regular clasp has a push-button system. Luckily, the 42mm Mare Nostrum also has 20mm lugs, so straps and clasps intended for 40mm models will also fit this watch. For 2002 there is a new design of clasp out; sorry, no pictures yet.
As of the 2002 collection, most "contemporary" 44mm watches also come with a deployant clasp.
Panerai also make some 40mm and 44mm bracelet watches. Bracelets and straps are interchangeable on 40mm watches, however a bracelet 44mm watch cannot be retrofitted to a 44mm watch that was only ever intended for straps. The bracelet is all steel (and very heavy!) on 40mm watches, but has alternate steel and titanium links for 44mm watches.