We have tunneler's friends. Or failing that, storage seals filled with fresh air in wooden crates.
E: OR! We put tiny air holes at the very bottom of one wall. Not enough space to get lots of water through quickly, and if they do try getting water through, it won't go very high. Also put purifiers angled to catch poisons they try to get through.
Or don't.
I can see why the other Leaf ninja would hate this idea. I hate this idea the more I think about. It completely cedes the initiative to the enemy. They get to stand around trying whatever shit they want, and the team just sits there trying to tank it with no means of fighting back.
A fort, as in a few walls strung together, is a fine idea as long as it's just a place to give wounded some cover. To helps keep chakra beasts from crawling out of the water and biting your fingers off as you sleep. But no, not this. Never this. Fuck it, I'm going to openly oppose this plan with one that doesn't do that shit. Hold on for Plan "No Hill Fort", coming soon.