It's the reflection. I only used a default shader (with metallic settings, translucency, a bit of luminosity and color input) for the whole sea, with just a small local area of real reflection (masked by sss) in front of the shipwreck. No transparency either, but softly masked ground colors fed into the color input of the default shader. Saves an enormous amount of render time (I'd say 75%). And it makes far off reflection less 'busy'. But of course lacks reflection of mountains, trees and clouds, though they are often only distracting in semi-wild water anyway. So I like this compromise for certain settings.
