Simple Dive Gear

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.