As all boat owners know, salt water is hell on metals. Saltwater reels are typically made of graphite, plastic, stainless steel or anodized aluminum. These are corrosion resistant materials, and they work. I would, however, watch out for the use of regular aluminum in combination with stainless. Electrolysis will eat this type of construction unless you keep it rinsed, oiled and clean constantly.
Frame construction is also a factor.
Older-style reels have posts that connect the two sides of the reel. This works fine, but such reels are harder to take apart and reassemble for servicing. Also, with big fish, these post frames can get torqued. The reels used for heavy applications have one-piece frames. The interior construction of the reels is also critical for performance and long life in salt water.