Dive

In the Version 2.0 update, the ability to dive underwater and swime was given to all players after helping Manaphy in Bleak Beach. This mechanic is different to the usual movement and has you able to go up and down with the shoulder buttons, or swim by holding B.

In addition to this, you have the ability to go to Bubbly Basin which is almost entirely underwater, and so various mechanics exist. If you go too deep underwater, it will go completely dark requiring illumination to be able to see.

You can grow crops underwater, but you need to have a Pokémon with Water Specialty, or use a Bubble Machine, to make bubbles to surround the plants else they will not grow.

You can build at various levitations without a connecting platform if you have buoyant blocks. These can be placed wherever in water and can then be built upon.

If you eat a smoothie, you will be able to have boosted Lapras power. While it lasts, you will be able to swim as Lapras underwater meaning you can go a lot faster<. Using its abilities, you can also break rocks using Rock Smash while you're transformed into Lapras. This can be charged up to take out multiple blocks in a row.