1. It is possible to get the last star coin in World 1-4 as Super or Fire Mario. Just slide down the slopes to gain enough momentum while crouched.
2. The blue shell powerup allows you to swim more easily (without it, it is difficult to change direction while swimming).
3. The blue shell can make you invincible to many enemies including sushis (the blue sharks), which can go through you in your shell form.
4. Otherwise, it will act as a block for some enemies like goombas and koopas.
Below is a video I recorded of my gameplay:
Spoiler: show
