Core problem: You have the Expansion Pass (and/or 2.0) but still cannot get underwater into Bubbly Basin, or you got in and building is a mess because water keeps waterfalling through windows and roofs. Players keep hitting the same two walls: missing a Bleak Beach prerequisite, then missing the underwater gate behind the Pokémon Center.

1What You Need Before the Gate Works

2Get Dive, Then Enter Bubbly Basin

No extra Trainer Level is required. Only Dive plus Expansion Pass. If Popplio is not following, talk to them first so they tag along.

3Once You Are In: What to Do First

Key Decisions

DecisionWhat Players Recommend
Buy Pass vs just 2.0?2.0 gives Dive plus underwater building in old water. Pass gives Bubbly Basin town, new mons, buoyant blocks, outfits. Gate will not open without the Pass.
Build on seafloor vs buoyant blocks?Buoyant blocks let houses sit at different depths. Use them if you want stacked layers instead of one flat seabed town.
Fire or furnaces underwater?You cannot light a bonfire underwater. Furnaces and furnace generators can work with Frillish or Jellicent. Torches are inconsistent when fully submerged.

Common Mistakes to Avoid