Make sure the finish you use contains a mildewcide. Both are formulated to seal out the elements. As their name implies, clear sealers are nonpigmented finishes. Unlike paints, which form a surface film, clear and pigmented finishes penetrate the wood and can stand up to foot traffic. It repels water, preserves the wood with a mildewcide and screens out UV rays.
Some products waterproof only.
For maximum protection, the label should list all three features. Which you choose depends on your priorities. Familiar names here include Cabot's, DAP, Flood, Olympic, Thompson's and Wolman. Water-based finishes also last longer than they did just a few years ago. Clear deck finishes are popular because they allow the natural grain of the wood to show through.