Water Damage (I can't afford repairs or a new phone)

Hopefully you have waited a couple days to let it dry it. If it hasn't been a couple days, turn it off and let it dry out. The problem may fix itself (especially in terms of the silent mode switch glitching).

That being said, the bare-bones cheapest thing you could do would be to use some some cheap $3 earbuds with the $9 Apple Lightning port to 3.5mm adapter: https://www.apple.com/shop/product/MMX62AM/A/lightning-to-35-mm-headphone-jack-adapter

This wouldn't cure your silent mode to not glitch, but at least you could make phone calls.

I would consider if you might be able to get one of these lightning jack adapters free from a friend. They used to be included standard until the iPhone 7 or 8 I believe, so lots of people have ones laying around in drawers they never used...but even from Apple the are cheap.

Given the restrictions you provided in the question, Nothing. An iPhone 6S in the US with the out of warranty replacement fee is $299. That is the cheapest you will get from Apple.

I'm not sure what you hope we can do for you? iPhone is NOT waterproof and a phone as old as iPhone 6s less so. If it's water damaged and you can't afford anything, there's nothing anyone here can do for you.