TNT causes environmental damage even in water

So for a time I had fun with Icarus Cannons (aka Player Cannons). 10 TNT behind the player are set off at the same time, water will flow through the primed TNT as always and the player gets shot into the sky, leaving no damage to the cannon or environment.

However, ever since a few snapshots ago, TNT still causes damage to the environment. Sometimes it's seeable: the TNT hops onto the slabs the player is supposed to stand on, and after the explosion a huge crater is left. Even if the TNT remains fully surrounded by water, everything around the cannon will explode.

Steps to Reproduce:
1. Build the TNT cannon like the one that's attached to this issue. Other TNT cannons may also be bugged.
2. Activate the cannon to ignite the TNT.

I believe this bug should be fixed as quickly as possible, as it affects common Minecraft TNT sense and leaves behind a huge mess. Also it takes away some of Minecraft's most fun game physics (TNT cannons of all sorts).
This bug did NOT occur in 1.7.4, even though everything is done exactly the same.