This mod is for Fabric only.

What is this mod?
Better Swimming offers a subtle quality-of-life improvement to the water mechanics of vanilla Minecraft. Water blocks now allow light to pass through fully, causing the ocean floor to be illuminated by sunlight during the day; and several other base swimming mechanics have been improved to make being underwater more enjoyable.

Better oxygen system

Longer breaths - 80 seconds instead of the original 15 seconds (1600 ticks instead of 300 ticks)

Drowning is similar to Subnautica'a system:

As you near drowning the screen slowly fades to black over the course of 5 seconds
Once the screen is completely black you still have 4 seconds before you will actually drown
Drowning is instant death instead of the usual 2 damage at a time

More responsive and easier-to-control swimming

No passive sinking - your character automatically treads water for you
Wading has a speed buff, so it's the same as walking on land
Swimming (sprinting underwater) is also about the same speed as sprinting on land

Easier to mine underwater

No mining penalty for being underwater (and, as a side effect, no penalty for mining while jumping/falling in air)

Nicer to look at

Water no longer subtracts light levels, so ocean beds will be illuminated from sunlight during the day

Ocean floor during daytime, viewed from above Underwater mountain range during daytime

Screenshots were taken with the Complementary v4 shader.
This is my first published mod so fingers-crossed it works well.