A waterproof VMU memory card
A 14-inch CRT display
A copy of The Ocean Hunter
A 1000BASE-T network adapter
 
Correct Answer: A 14-inch CRT display

A little background…

Designed as a joint project between Sega, CSK and Fuji Television, the Divers 2000 Series CX-1 was an iMac-esque device that featured a Dreamcast inside a 14-inch CRT TV. Only 200 of the systems were made and those were only ever shipped in Japan back in 2000.

The Divers 2000 CX-1 was sold for about four times the cost of a standard Dreamcast ($199 at launch) and it included additional accessories including a DreamEye camera, a specially-branded Dreamcast controller, a keyboard and remote control for the TV. Speaking of which, if you're interested in buying one of these for your collection, make sure it at least has the TV remote because you need it to switch into Dreamcast mode.

Incidentally, The Ocean Hunter wasn't a Dreamcast game but ran on the Sega Model 3 arcade system board and has since been ported to PC by fans.