The one item I use most often to save a dive, either for myself or a buddy, is a used Pressure & Depth gauge. I picked up a used one from my LDS when they were upgrading all their rental equipment for $40. It’s big and not pretty, but it always works! Most of the time if my computer battery dies I have a spare battery. But not always. And many a relieved divebuddy has been grateful that I had a spare for them to borrow. Hand tight works. Don’t even need tools.