Are you positive that 100% of the leaks are above the existing water line? For example, your pond could be tied to the ground water level. No amount of compaction and sealing above the water line will "fix" the problem if your pond is still connected to the ground-water table.

If you are sure you are correctly addressing the problem, then I think you should probably just "go for it" with the best available means of compaction at your disposal.

One of my most common "errors" on projects is to suffer from "the perfect is the enemy of the good" syndrome. I over-plan, rather than just performing the work, figuring out what went right (and wrong) and then better fixing the problem on a minor second step.

I usually determine that there was no way to get it perfect on the first try, especially for a new project that is NOT in my everyday job expertise.

That being said, yes you do need water on your soil to optimize your compaction - regardless of your method.

If your soil is still "fluffy" from the tiller, then I would water it lightly (like misting) to set your bentonite in place, rather than have it run down the slope. I would then keep watering to try and get your soil wet below the level where you have tilled. You will probably have to do that in small steps.

Without deep disking, or compacting in lifts, you are probably not going to get any significant compaction below the 3"-4"(?) that was well tilled. However, if you compact that layer very well AND your bentonite performs well, then that may be enough to fix your leaks.

Good luck on your project!