Minecraft wiki entry for Igniting_TNT_underwater, associated text: Unlike the previous method, this method doesn't require the use of glitches to function. Surround the TNT by blocks with low blast resistance, or dig a one-block hole in the ground and place the TNT inside, so that no sides of the block, except the top side, touch water. Then cover the top of the TNT with a redstone block to activate it. Redstone block was chosen because it is the only source of redstone signal to function underwater, not counting the USB Charger Block.