Act 2 increases the complexity of the underwater maze. You'll encounter more elaborate sequences of crushing blocks and strong currents that can push you into spikes or other hazards. There are also sections where you need to push blocks to activate switches or clear paths. The Jaws enemies are more prevalent here; they are fast and can be difficult to hit underwater. Try to Spin Dash into them when they are vulnerable.
A significant challenge in this Act is a long vertical ascent through a water-filled shaft with numerous spikes and crushing blocks. You'll need to use springs and careful jumps to navigate this section, constantly looking for air bubbles. There's another 1-Up Monitor: after a section with a series of vertical springs and crushing blocks, you'll reach a platform. Instead of going right, look for a small hidden passage to the left, which leads to the extra life. This Act demands extreme caution and excellent spatial awareness.