Iscalon has no indoor structure to explore. The entire stage is comprised of swimming through water. Combat should be kept to a minimum here; if you have 99 Force crystals, there's little benefit to fighting enemies. Try to swim through most of the stage by avoiding enemies unless you are in need of more crystals.
You begin on the left end of a long lake. You can actually cover a great deal of distance simply by swimming along the top. The first fish that you encounter will be swimming at the top of the water, but all subsequent fish will be below you. Continue right until you reach a set of pillars with gaps in them, surrounded by spiked blocks. Carefully maneuver your way through the gaps. When you reach the wall of Force blocks, you can either punch a hole in the middle and swim through, or swim below it. You'll have to pass three jellyfish, and then you'll see a large fish swimming above spiked blocks near the floor. Avoid both and follow the flow of the terrain until you are swimming down into a deep chasm.
As soon as you start drifting down, you will see a small tunnel cut into the right side of the wall. Break away the blocks (and watch out for any squids that come swimming upward at you) and swim through the tunnel. You'll be in a room with four jellyfish bobbing up and down and an extra life capsule on the right side. Swim to the capsule and collect it, and then return through the tunnel back to the chasm. Once you resume your dive to the bottom, you may wish to consider using Force Invincibility. If you do, you won't have to worry about the squid that appear quite rapidly when you're diving down at full speed. If you don't use it, be very cautious and try to judge the position and the direction of the squid before you crash into them.
As you float down, you'll see a couple more fish. When you reach the bottom, the tunnel will resume proceeding to the right. The blocks in the area put you in more cramped conditions with the fish swimming around, so move carefully. Two particularly big fish guard the entrance to a building, the first fish swimming high, and the second fish swimming low. Watch their movement and attack them from a safe distance, or attempt to swim through them when they create an opening.
Once you're inside, you'll be face to face with another "Darth Vader" clone. As usual, you must strike this one once before it reveals its true form, which in this stage happens to be a very large fish. This is actually one of the easier battles thanks to the fact that the fish will not occupy the highest portion of the pool. You can safely stay at the top and wait for the right moment to strike. The fish will swim to one side of the room before turning around and diving down to the bottom. The best time to attack is when it returns to the top, and you can chase after it and strike its tail with your lightsaber. Four hits will defeat it. Then collect the large crystal, and exit through the door on the right.
You'll find C-3PO. Don't forget to wave your lightsaber once to remove the beams before attempting to rescue him. Once you return to the Millennium Falcon, you'll receive a distress call from Princess Leia herself. It's off to the Death Star. You'll have to defeat nine enemy fighters before you can approach it.