If you've been searching for as long as I was (a long time) to find something for you server that would allow TNT to blow up water,
yet NOT completely ruin the ability to make & use TNT cannons, then you might not be believing your eyes right now, because
this plugin does exactly that!!
For as long as there has been TNT players have been making cannons- using the blast from some number of TNT to fire one or more other TNT to a more-distant location. (Search 'TNT Cannon' to learn more about what a TNT Cannon is) Of course, as anyone who has messed around with TNT explosions knows, all TNT block damage is cancelled out if the TNT detonates in water.
This can make TNT less useful against water-logged targets. However, most TNT cannons require water or some other liquid so as to prevent the cannon destroying itself. This plugin is the solution to this problem! TNT will, by default, have the ability to destroy liquid blocks with this plugin, but it will not do so unless the location at which it explodes is at least 15 blocks from the location at which it was lit.
This plugin is all about simplicity and efficiency; hence, only one command:
/h2oTNT will toggle the plugin on/off, default is "on" (configurable)
Once it is toggled on, TNT lit by any player will have the potential to blow up water
