The dive band kit is what turns a Descent X50i from a watch you wear on a bare wrist into one you can strap over a wetsuit or drysuit sleeve. It supplies the longer band hardware for that job, so the computer stays flat and readable through the dive instead of sliding round the arm.
It is a genuine Garmin part made for the X50i specifically, which matters on a dive computer: the mount has to hold under the drag of moving water and the compression a suit goes through on descent. Keeping a dive kit fitted and the standard band in the bag is the simplest way to run one computer for both diving and daily wear.
