Under Looks Best
A deck that terminates just under the top rail of an above-ground pool looks best because the deck edge and ends of the decking are hidden. Use this if looks are more important than functionality.
Over Works Best
Terminate the deck on an above-ground pool over the top rail of the pool if your pool is destined to have lots of people running, jumping and climbing in and out of the pool. A deck built above the top rail will eliminate the problem of stubbed toes and offer protection to the rail as people climb out of the water.
Decks can be built to go over or under the top rail of a pool. Heavily used pools should always have decks higher than the rail. Most pool decks are built this way. If appearance is of paramount importance, a deck terminating under the rail gets the nod.