I'm willing to bet you would have never seen that honestly. That looks like it started as a very hairline stress fracture that turned into complete metal fatigue.
I am surprised to see two mishaps like this on boat trailers. In the offroad world, the leaf spring crowd can break them often, but we are talking about a factory spring that was used and then salvaged to use on a purpose built crawler. Years, maybe decades of use, then used exclusively over an axle that over flexes the springs in both directions for who knows how long before breaking.
Just makes me curious of what happened to that spring. That looks like nothing I have ever seen as far as leaf spring breakage. Whether it be mfg. defect or what I do not know, but it just doesn't seem normal.