Metal with say a "pool rug" would be better than wood just because when people get out of the pool they would drip water all over the wood and if the wood was not sealed it would rot. You best bet would be either metal or composite wood, both are much easier to take care of around a pool envorinment
Metal decking is a long-lasting and generally maintenance free type of outside decking. This decking will sit a top wood or metal joists and individual pieces lock or snap together to create a close-fitting deck material. Metal decking is usually made of aluminum, but steel can also be used.