Enhancement
The replacement player model when sprint swimming currently has no idea what Armourer's Workshop is. I've been told adding support for AW cosmetics isn't all that hard, but I suppose if I understood it I'd just file a pull request after doing so myself.

Versions
Better Diving: 1.5.0
Armourer's Workshop: 0.50.1.589
Forge: 14.23.5.2852
Minecraft: 1.12.2

So I removed the rendering of a custom player model in v1.5.1. Now it should work fine with Armourer's Workshop (maybe you can give me feedback).

Everything looks swimmingly now. I've repeated the tests with both Armourer's Workshop and attempting to swim with a giant player and everything looks amazing!

Additionally, I double-checked with Extra Bit Manipulation which also now works well too. I'd call this one complete!