Take this all with a grain of salt as im not too skilled at ue4, and i don’t exactly get the question.
For the upper part/outer layer: Just use simple opaque/masked materials unless im misunderstanding completely, easy peasy.
“volumetric mesh”: Not even google or wikipedia knows what this means, almost definitely not supported, on all engines, likely forever. Unless your just asking for a simple volumetric effect, then try modifying one of the local volumetric fog tutorials, or are asking for a secondary layer inside, then ignore all of this and just use 2 materials, 1 inside 1 outside.
“that is manufactured as a procedure in run time and constantly updated”: If there is any alternative with an .obj or and .fbx, that is your best bet, as runtime is mostly plugin-dependant here as far as im aware. And updating it is likely easy with blueprint and materials.
“As we move from the top to the bottom of the mesh, its stiffness is reduced and looks like a transparent jelly body and passes through the light.”, People struggle with getting water opacity right, to such a degree finding a shader that doesn’t just make it opaque, fixed opacity, or depth fade is incredibly rare, and volumetric lighting requires either modifying the engine’s source, a hack, using some sort of volume material, or being really, really smart