An underwater Smoking Block as seen from above water.
A Smoking Block (henceforth known as a Smoker) is a breakable block that emits a semi-view obstructing smoke. It is part of the Twilight Forest mod, and cannot be crafted.
It has its own physics, for example, when placed underneath a canopy of some sort, say, a tree, the smoke will billow out and around it, at least to the end of its range, which is approx. 30 blocks, straight up. It is not able to be controlled by redstone. It will become visible within 60-61 blocks.
The Smoking Block will work underwater, but you will be unable to see the other half depending upon your relation with water.