regarding beam height, I used 6" over normal pool. I know of no standard distance, but 6" seemed to be reasonable to avoid the tallest possible wave that a high wind might produce in our BOW. In the end, 6" could have been overkill and 3" might be just fine (assuming you are using treated lumber for the framing). Lower is always best, particularly when drought drops the normal pool level significantly.
Regarding the hidden fasteners, I see a need to replace a decent decking material as a minute percentage of risk. Even then, you would cut it out in similar fashion to removing a damaged section of hardwood flooring....by cutting the length of the damaged section with a circular saw and working off both pieces from the center at the cut. To replace, you would either have to resort to face screwing (screw heads could be countersunk and plugged if you could not accept the look), or use another type of hidden fastner system that is mounted from the bottom (a galvaninzed mounting bracket system that will require access from the bottom of the deck to set the final screws that pull it down tight, readily available in many home improvement stores)