Here is a picture of my Trailex trailer underside as I was assembling it.
The axel & the springs are ordinary steel and will rust. These parts bear the most weight and must be strong; hence, ordinary steel. They have been painted with some rust resistant paint, but its effect will be minor. The bolts are also ordinary steel and coated with zinc--they will resist rusting. The slab springs are particularly susceptible to rusting because saltwater gets between the spring sections and cannot be washed out by any simple means--dunking the trailer in a freshwater pool might do it, but that is inconvenient.
The aluminum frame will not rust; therefore, it is your choice if you dip your Trailex trailer in saltwater: plan to put up with the inevitable rust of your axel, springs, and, to a lesser extent the zinc coated bolts--these can all be replaced after a few years relatively easily.
Some people don't expect these things to last forever and are comfortable dipping them in saltwater. Maintenance will be relatively minor for 5 yrs. The springs will likely go first. Personally, I don't like to spend my time on a lot of maintenance--I did that for years when I had a powerboat on a galvanized trailer. Therefore, prevention
is my motto, and I don't dip my Trailex trailer in salt water. Of course, it also makes a difference whether you have an AI (easy to load) or a TI. I have a pair of AIs.
Oh yes, the lights will probably go in 1-2 yrs; maybe they will last 3 yrs, but don't expect much. Many people, after the lights fail, build a "light bar," which is never submerged in saltwater.
You can see the description of my Trailex trailer construction at http://www.hobiecat.com/forums/viewtopic.php?f=70&t=7276&start=435