I too have had my leaf springs break, never had that happen on any of the other 14 boat trailers I've owned for past 40 years. Mine happened on my way "up" the Grapevine (Interstate 5 North before Bakersfield). I have always washed down my springs and brakes along with the boat with fresh water right after retrieval from the saltwater. I didn't freshwater rinse them after launch generally, so I was only doing maybe 50% of something right.
On this boat (CD25) when it was new in Dec' 2002, I coated all the visible surfaces with wheel bearing grease, thinking it would help preserve the non-galvanized metal. In 2009 (I think), that's when they failed. I hit a pot hole on the I-5, it broke 1 or 2 springs after I inspected it off the highway. I figured I could make it to the Pacific Trailer factory near San Jose if I drove under 50 mph, I did. But when 1 or 2 springs break, the others are under more stress. I had one (1) spring left on the curb side when I arrived at the trailer factory. The guy just shook his head, we all new it was stupid to have continued on that far. He said I got "good" service from them, usually they're shot in 4-5 yrs! Geez. He said there's really no coating or other method to preserve them-- I just don't believe that. On a previous boat trailer, when new, I coated the springs with a tar-like, spray-on substance very thoroughly, didn't have a problem in the 8 yrs I owned it.
BTW, my trailer is an oversized 27 ft, rated at 11,500 lbs, total weight boat and trailer normally loaded is 7,400 lbs. So I shouldn't be over-stressing the springs, bearings or tires (well maybe I did with that pothole...). Maybe spring life has to do with the quality of the metal used. These seem to be doing much better, very little rust showing, more of a surface rust film.