There isn't any way to shut the Cobalt off completely, short of removing the battery. Power is required to maintain the real time clock, though the drain is very small. And the Cobalt is always ready to go into dive mode if it contacts water, even if it is in sleep mode.
You should be able to go at the very least 3-4 months on a full battery before charging is needed, and in sleep you should not see a decrease of more than 1-2% per day in the charge state. It's best for the battery to not let it drain all the way to zero, so we recommend periodic charging. If you are not getting that much standby time The battery might be starting to decline in capacity.
Since Atomic no longer maintains their forum on ScubaBoard, I have put up a manufacturer's forum section to support our computers here:
Ocean Concepts
-Ron