Every time you surface with six divers, the value of enemy subs and killer sharks increases by 10, up to a maximum of 90 points each. Rescued divers start at 50 points each. Then, their point value increases by 50, every time you surface, up to a maximum of 1000 points each. Also, you'll be further rewarded with bonus points for all the oxygen you have remaining the moment you surface. The more oxygen you have left, the more bonus points you're given.
Difficulty is called skill, and a is expert, b is novice.
Your Oxygen Gauge is at the bottom of the screen. Whenever your sub is underwater you'll be using up oxygen at a constant rate. When the oxygen tank is almost empty, a continuous beep alarm will sound and the alarm light will flash. If you don't surface in time, your sub will blow up and you'll lose one diver. Each time you're forced to surface, with less than six divers, you lose one diver.