This level is a lot easier than the ones before it. The level is divided into four areas, each with a single entrance, though this time you will need to jump over spike hazards on the ice. Due to the numerous enemies and the relative scarcity of air, you'll want to Dash for most of this level. Fortunately, fish are plentiful, and you'll want to grab them whenever you can.
From the starting area, you'll need to swim down past a large number of crabs, and then to your left, pushing upwards through a shaft filled with Arctic jellyfish. It may be a little tricky getting out of the starting area, as there is a downwards current; it seems to be weaker on the left hand side, and you'll need to head left once you get out. It may help if you Accelerate upwards and then Dash diagonally up to the left when you're near the surface. Once you've got to the leftmost pool, heal yourself if you need to and watch out for the lurking Arctic spider crab. You'll have noticed a Key Glyph above the pool, and you need to jump at it to activate it.
Once you've done so, you'll need to make your way to the area to the right of where you started. This is where you'll need to start jumping over spikes. You should be fine if you jump as soon as you land, but if you get hit by the spikes they'll hurt you and send you the other way. You may accidentally miss the entrance, and end up in the rightmost area. If you do, avoid the Arctic spider crabs and slider back over the ice to your left; you don't need to worry about hazards from this side.
Once you've made it, go down, then right, then swim up and turn right at the fork. From here, you'll need to swim down and sing to get rid of the Barrier Glyph. Continue downwards, then head left, taking the lower path. You'll pass another Key Glyph, which you will need to activate to get out. As you continue to the left, you'll see Big Blue, and you'll realise why he's called that. Get in front of him and sing to him, and he'll provide a lot of exposition: The short version is that your new goal is to find the Asterite.
Head back the way you came, making sure you activated the Key Glyph, since you'll need to pass the Barrier Glyph again. Once you're past it, head left and down, then surface for some much-needed air. Like the first hole, this one has a downwards current, and you'll want to make sure you've dispatched the Arctic spider crab before attempting to jump out. You'll need to slide right across the ice, and land in the final rightmost area. From here, go downwards and turn right once you see a turning; this is the entrance to the next level.