Cairns is the busiest reef gateway in Queensland, and that cuts both ways. You get the widest choice, the most departure times and the lowest prices — a half-day fast-boat snorkel starts at $128 against $236 for a small-group day out of Port Douglas. You also get forty listings that read almost identically.
Three questions separate them.
How long are you actually at the reef? The day length is close to meaningless. A Cairns full-day boat spends three hours of its eight to nine steaming, so what matters is the in-water figure. Down Under Cruise and Dive states five hours across two exclusive moorings — the highest we found. Silverswift reaches three sites in a day, which no other Cairns day boat does, by being the fastest dive vessel out of the marina. A half-day trip gives you about 90 minutes at one reef, which is genuinely enough if you are fitting it around a flight.
How many people are on the boat? This ranges from 27 on a Reef Adventures fast boat to several hundred on a large catamaran running to a permanent platform. Neither is wrong — the small boat gets you quieter water and a crew that learns your name, the big one gets you a semi-submersible, an underwater observatory and somewhere dry to sit.
Are you diving? If yes, the boat matters much more. Dive-focused vessels like Silverswift carry the instructors, the gear and the site rotation to run up to three certified dives in a day. If you have never dived, an introductory dive on any of these boats is added at booking, needs no certification, and puts an instructor in the water beside you the whole time.
One Cairns-specific detail worth knowing: several operators collect a fuel surcharge at check-in on the day — typically A$10 an adult and A$5 a child — that does not appear in the online price. It is small, but it is cash at the counter when you are expecting to have paid already.