Most of the dives are of the type where you go down, stay put, and watch the parade go by. Then you might move to a different spot - ie, a second or a third cleaning station - or if you are shooting and want to shoot from a different vantage point. The remainder of the dive is usually a drift with the current, eventually ending in blue water to do the safety stop. On occasions, there is a little bit of swimming against the current to get to a particular spot, or if by luck, something big swims by that you may want to chase.
Training to improve fitness and stamina - whether in the pool or in the gym - will definitely help, given that doing up to four dives a day over a span of a week or more can be physically draining, regardless of your age or conditioning.
However, dealing with the other potential challenges - at least in my opinion - is more mental than physical and has more to do with underwater experience under different conditions, such as the examples listed in my first reply: strong currents or surge, particularly up or down currents, sudden changes in temperatures, the possibility of getting blown away and separated from the group and surfacing under choppy conditions, etc.
These are very real situations that don't necessarily occur on every dive or even on every trip, but the possibility of them happening is much greater in a place like Cocos than let's say in a place like Bonaire. How you react to these situations - were they to occur - will have a significant effect on the outcome and enjoyment of the trip not just for you but also for the rest of the divers on the boat.
The thing is that I don't know that these situations can be easily replicated in a pool or in a gym such that you can train for them. And 200 dives under relatively placid conditions may not get you there either. Rather, I think that it is thru the accumulation of time underwater under increasing levels of difficulty that you will gain the necessary underwater experience.
When you choose to go is ultimately your decision. I've seen newly certified open water divers dive the Galapagos (similar and sometimes worse conditions) without a problem - luckily for them, the conditions were favorable. But I think that in the final analysis, most people will enjoy a place like Cocos a lot more once they have gained some experience in dealing with adverse conditions without panicking, in addition to having good control of their air consumption and buoyancy.
