The problem with translucent materials underwater is because of the single layer water (SLW) materials. Apparently there is no way to solve this as this is how SLW works in the engine because of its priority in the redering pipeline order. At the end of the day it is nothing but an opaque material which is giving a mirror image of the other side. The translucent object would pick its “color” and modify it but instead it takes the mirror image resulting in no visual change.