Yes, it seems that activated TNT does not flow in water anymore. However, with a tiny bit of rewiring, the problem can be easily solved. The solution is to use a single TNT that detonates a second or two before the rest to push them near the TNT that is the projectile. This can be done by running a redstone wire to the TNT farthest from the projectile. This wire can be connected to the trigger, just like the wire that activates the rest of the TNT. The key is to use a repeater or two to act as a delay so that the single TNT detonates first, pushing the rest near the projectile. Those then detonate a second or so after, launching the ammo nearly as far as when water was used to push them.Here's a diagram:
[] [] - the TNT in this row is one higher than the rest and is the projectile.
[] [] - the redstone in rows 2-6 are on top of stone.
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[] [] [] [] [] -these sticks are repeaters [] [] [] [] [] [] [] [>>-i>] -this is the trigger, (button, lever etc.)
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