A dog fed two big bones has Fisher Dog maxed out and Swimmer Dog at 1. It can't actually breathe water while ridden and instead drowns. It also doesn't seem to get its movement speed buffs in water (or is crazy-slow in water by default) and can't climb out of even 1-block high water without help. Also, while ridden, it not only can't climb out of 1-block high water it drowns in it.

Is Swimmer Dog somehow blocking Fisher Dog's mastery effect? Would it be possible to move ALL the breathing stuff to Fisher Dog and make Swimmer Dog improve swim speed?

Hey, thanks for taking the time to create this issue and I'm sorry I didn't get round to resolving it. I've not worked on this project for a while and have decided to archive the git repository. Fear not, there is a fork https://github.com/DashieDev/DoggyTalentsNext which might be of interest. I don't expect to be back within the Minecraft community so farewell and happy mining!