An unprotected deck will deteriorate quickly. Even though pressure treated wood resists rotting and insect predation, it will still crack and split from water exposure. The only way to protect your deck successfully over time is to apply a deck preservative. There are sealers, stains and paints on the market especially designed for deck use. Aesthetically, they look very different, but they protect wood and composite decks from moisture damage, fungus growth and ultraviolet light. Some also have built-in fire retardants.
Deck protection products are effective, but they have one big drawback: They lose their ability to protect wood over time and have to be reapplied. Usually, the more expensive the product, the longer it will last between applications, but there's no magic bullet that will offer lifetime protection for a wood deck. The most important thing you can do to protect your investment is to reapply a wood sealer on a regular basis, typically in fall when the temperatures are stable and rain isn't forecast for a week or more. Once a year is considered pretty standard, but newer formulations may reduce the reapplication frequency to once every three or four years -- if you're lucky.