Making TNT explode underwater seems impossible, especially if you're not using the Education Edition. However, with this special glitch, you can create explosions even though you're in water!
- 1 sand, gravel, or concrete powder
- a flint and steel or fire charge
- any kind of building block, but not too expensive (recommended, to reduce the fail chance)
- Find any body of water. It could be a lake, river, or in the middle of an ocean.
- Place the TNT block, with a block of sand or gravel on top.
- Ignite the TNT with your flint and steel then step back and watch the magic happen. (Note that the TNT would move slightly when it's ignited, hence a few blocks are needed to prevent it from happening.)
A more expensive, but simpler, method
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.