Customer retention can be one of the 'magic bullets' that changes you from an average dive centre into a sucessful one. Any customer that keeps coming back to you makes one less that you have to find, and the cost of bringing the old customer back can be a fraction of the cost of finding a new customer.
Any dive centre with good customer retention will find it can spend less per head on marketing and promotion, making each customer (returning and new) more profitable. Good customers will also help you promote yourself in many ways - they will recommend you to their own friends; they will put good comments about you on message boards and in chat rooms, and defend you in any relevant discussions. All of this free publicity is a great help to your business and should not be underestimated.
So, what can you do to make customers come back? The simple answer is give them the sort of service and diving experience that they want. However what exactly do they want? To answer that in detail the best way is to actually ask them. Put together a simple customer service questionaire and find out what makes them tick. If you can find the relative importance of different elements, then you know where to spend your time and effort improving 'customer service'.
Set their expectations at a level just below what you normally deliver, that amaze them by delivering your normal service!