The ferry ride to Manly is genuinely one of the best things you can do in Sydney. Thirty minutes of spectacular harbour views, passing under the Bridge and along the heads, then you step off into a relaxed beach suburb. From there you can easily walk to the snorkel sites. The underwater scenery isn't the Great Barrier Reef, but it's surprisingly good for a city location. Expect to see lots of fish, rays, maybe a Port Jackson shark, and healthy kelp gardens. The water is usually clear enough that even beginners have a decent time. Tours typically combine a short guided nature walk with 1–1.5 hours snorkelling in a small group.
Best time is December to April when the water is warmest (18–22°C) and visibility is better. You can still go from May to November but you'll need a thicker wetsuit and the experience is colder. Expect to pay around A$120–190 per person for a half-day small-group tour including snorkel gear, wetsuit and ferry transfer. The ferry itself is cheap (around A$10 each way with an Opal card).
Pick a small-group eco-focused tour if you actually want to learn something and stay in a group of 8–12; they're better than the big generic ones. Skip the super-cheap no-guide “just rent gear and go” option unless you're an experienced snorkeller who knows the local rips and protocols. The harbour can have strong currents near the heads.
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