1. TNTs won't explode underwater, but the "pushing power" things still exists, so you put water flowing from there:

Here's how you load it:

I have redstone dusts attached to 1 lever, i push the lever so the 6 TNTs will soon explode, but before they explode, I go hit the other 2 TNTs that I didn't activate with the lever:

The last 2 TNTs that I activated will fly and explode. This is what the damage is to the environment: