Deck are a major investment and maintenance is critical. Water or oil based coatings I am often asked which is better. I have seen great results with both and it is personal choice as to which way you go.
Build up of dirt, moss, mould and lichens all speed up timber degradation, regular cleaning will even help reduce damage.
An example of degradation in treated pine decking in the Adelaide Hills. Regular oiling would have slowed the end grain rot. This area of the deck is unstable and needs replacing.