Imagine the following situation:
You successfully max out your reputation with the Deep Ones, and you obtain a Magic Conch. That's great! Now you can get a massive helping hand on bigger battles.
You summon them using the conch during, let's say, an Illager Raid, and charge, but one or two of them get in the way and you accidentally hit them with your sweeping edge sword. Now they no longer regard you as a friend and leave you to fight alone.
I have a very basic suggestion to fix this issue, which is, if you have maximum reputation, you only lose those points upon killing Deep Ones, instead of upon harming them. That'd make it less stressing, because as it is currently, I can't make a single mistake or I'll lose my friendship with the fish men.