Make sure to grab a Cannon Block from the start of the level. A little ways into the underwater section, you'll encounter some big blocks. Blow through them to reach the star on the other side.
When you reach the large spike blocks, swim upwards to discover a grotto. Avoid the Bloopers in this narrow channel, and you'll find a star at the end.
Now sink deeper into the water. You'll spot some big blocks by a sunken ship. Blow them away to find a hollow green star. Hit it, then collect the eight green coins that spawn before time runs out to earn a star.
The beep blocks in this course switch very quickly, so be on your toes; a Tanooki Suit is a good idea here. As you round the bend a bit into the level, you'll see the first star above a question mark block.
There's a hollow green star by the checkpoint. Hit it to spawn eight green coins, which you'll need collect within a time limit. Do this to earn a star; to reach the ones on the upper ledge, hop onto the blow platform and blow into the GamePad's mic.
You'll find this on the beep block slide leading to the level's end.
Head left from the level start to find a pier with four unlit torches. Hit the question mark box near the pier for a Fire Flower. Light all four, and a star appears. Alternatively, you can stand by the torches and get the Fire Bro. atop the Goomba stack here to light them.
Head back to the right. Hop inside the clear pipe here to reach a cloud platform above. There's a star up there.
On the far right side of the beach is a stack of three Goombas, all fast asleep. Snatch the start from over them.
Grab the Boomerang Flower from the question mark block at the level start - you're going to need it. Just past the two lines of Para-Biddybuds are the star and a Piranha Plant. Throw a boomerang over the gap to grab the star as it flies by.
Just after the spinning fire spokes is a clear pipe. Enter it from the right side of the pathway to pop out and grab the second star.
You'll find this during the final stretch of the level, when the hexagons are flying into place. It's floating solo at first, but then a lava piece connects underneath of it. Throw a boomerang to collect it the safe way.
The first star is to the left of the second Fire Bro. Cross when the spikes are receding, and use a Cat Suit to scramble up to the platform where the star is.
Just after the checkpoint, you'll come to a bridge with waves of moving spikes. Stop at the first pillar and take out the Fire Bro., then use a Cat Suit to scramble to the star on the side of the platform here.
Just past the bridge is a large platform almost entirely covered by a spotlight. Stick to the edges and move to the back to find star. This too has a spotlight on it, so be prepared to get out of there quickly or take the hit.
This level is just a gauntlet of previous boss battles, but you get stars for three of the fights. You'll get the first for defeating Boss Brolder.
The Hisstocrats actually fight together this time, meaning you have to hit each one three times to win. You'll get another star for taking them down.
And you'll get one more for beating Motley Bossblob.