I still believe that depending on water bottles and respiration 3 for going to nether is really difficult. I came to achieve it, but as well I lost my sanity. And you even mentioned me one comment once about using respiration 3 and water bottles, but I believe this is still really hard as you need to literally depend on RNG to spawn in a fortress. So I was thinking that an indirect buff such as making only water bottles stackable would be enough to not lose sanity through the whole process.

There's also the turtle helmet which helps with this.
Honestly, Merling's only weakness is their limited breath on land, and there are already soooo many ways to mitigate that weakness, that I really don't think making it even easier is a good choice. The fact that you can use water bottles also is more a gimmick than a real feature - it was added afterwards for "logical" or "consistency" reasons because people asked for it, without the intention of making it much easier for Merlings.