Wet Sponge is a Block that was added in Update 0.16.0. It can only be found in Ocean Monuments.
A Wet Sponge is an item that can only be found within Ocean Monuments. As the name suggests, it is similar to the Dry Sponge block except that it is saturated with Water, and it emits Water droplet Particles.
Wet Sponge is a block that appears whenever a Sponge is exposed to water. It will destroy the water (source blocks only) in a 5x5 square, centered on itself. A room full of 15-30 Sponge generates normally in Ocean Monuments; this will promptly become Wet Sponge and drain nearby water. Each Elder Guardian also drops 1 Wet Sponge, along with its Raw Fish, Prismarine Shards and Prismarine Crystals. However, this is not an efficient method of farming Wet Sponge, as the minibosses only spawn 3 times per monument and are difficult to kill.