I'm going to try to use minimal spoilers here. The game will tell you what I leave out.
If Surrounded Sea has an exclamation mark next to it, that is where you must go next. If not, skip the next paragraph or so, to "Miracle Sea".
Surrounded Sea is a Water dungeon, obviously. You may want to play using a Water Type or other Pokemon for traversing the Watery Terrain.
At the end of the dungeon(20th Floor), you will find a bunch of Deluxe Boxes and a strange-looking Egg. No matter what, grab the Egg!
Next, you'll need Blue Gummis. A fair few, I'd say about three. You'll find out why. If you don't have any, Beach Cave and Quicksand Cave sometimes have them, but it can be a pain searching for Gummis. Pick them up whenever you find one.
Next in the storyline, you'll have to go to Miracle Sea. The game will tell you when. There's a Boss Battle at the end, so be prepared. It's not very difficult, though, unless your team is very weak to Water.
After the subplot is wrapped up, play for three more filler days until you unlock the Team Charm arc(if they're in Explorers of Time).