As mentioned before,
this is the best result I had achieved.
But after testing the translucency on several maps, I've had one more idea.
Now I've modified the gamma correction algorithm on the blending, turning it into
adaptative gamma-correct blending. This helped to give the resulting image an even more natural look, with even more details being recognizable now. Both this screenshot and the previous one were taken with exactly the same settings (r_slime_blend_alpha 0.6; r_wateralpha 0.5).