Snorkelling is included on every reef boat in this catalogue, so the question is never whether you can snorkel — it is how much water time you are buying, and at what cost in day length.
A half-day fast-boat trip out of Cairns gives about 90 minutes at a single reef, within a four-hour round trip, from $128. A full-day outer reef boat gives three to five hours across two or three sites, for eight to nine hours and $197 upwards. That is the entire trade.
For a first-time snorkeller, 90 minutes is often the right amount. The first twenty minutes go on getting comfortable — clearing the mask, learning to breathe without thinking about it, working out that you float better than you expect. The next hour is the good part. Beyond that, cold and fatigue start to matter more than enthusiasm, and a wetsuit only delays it.
For anyone who has snorkelled before, the full-day boats are worth the extra hours, and specifically for one reason: the second site. Reefs are structurally different from one another, and the second stop of the day is where you notice it, because you are no longer thinking about the gear. This is also where guided snorkels earn their keep — Wavelength out of Port Douglas puts marine biologists in the water on every swim, which is why it rates 4.9.
Two practical things. Stinger suits are standard November to May and every operator provides them free; they also double as sun protection, which is the reason experienced snorkellers wear them year-round. And seasickness is the most common reason people cut a reef day short — if you are prone to it, take something before boarding rather than after, and pick a larger vessel or a shorter trip.
If you cannot swim confidently, this is the wrong page: the half-day fast boats list non-swimmers as unsuitable, and the small-group boats require a 50-metre swim. A reef platform day with a semi-submersible and an underwater observatory is the trip that suits you.