This mod adds a suit of deep sea diving gear (with optional upgrade for being able to see easier underwater), and a suit of lava diving gear.
When wearing the suits outside of their respective liquids, you'll be slowed and have a hard time mining, as it's quite cumbersome. But when in them, it's a completely different story. You get the ability to swim in place, quickly and reliably. You can break blocks at a more normal speed, and of course you can breathe underwater.
The suit is a glorified suit of armor with no custom models or insane textures to render. It's... Simple.
When wearing the suit out of water you find it's harder to move and break blocks
You can be underwater for a configured amount of time before you start taking air like the vanilla underwater mechanic
With the lighted helmet upgrade you gain the ability to see clear-as-day while underwater.