Act 1 immediately plunges you into the underwater environment. Your primary concern will be managing Sonic's breath meter, which appears as a countdown when underwater. If it reaches zero, Sonic will drown. Look for large air bubbles that appear from vents or small bubbles released by destroying certain blocks. You'll encounter strong currents that push Sonic around; use these to your advantage or fight against them with careful movement.
There are many crushing blocks and spike traps. Observe their patterns before attempting to pass. You'll also find switches that open gates or activate platforms. A hidden 1-Up Monitor can be found early on: after the first long underwater section with crushing blocks, you'll come to a vertical shaft. Instead of going up, look for a small opening to the left at the bottom, which leads to the extra life. This Act is a test of patience and precise movement, so take your time and prioritize survival.