Whatever happens, said Bec Finlayson to the audience in the top-deck lounge, don't come back from snorkelling saying "I saw a yellow fish", and expect her to name the species.
On colour alone, you might have seen a butterflyfish or rabbitfish or forcepfish or beaked coralfish, and even the Chinese footballer cod and bicolour angelfish have yellow bits, so you'll need to be more specific.
Finlayson, a marine biologist and general fount of watery knowledge, was delivering a crash course in fish, cetaceans, coral and climate change to the 20-odd guests cruising Australia's Great Barrier Reef marine park on the 35-metre Coral Expeditions II.
We were not long out from Cairns, the start and endpoint for the ship's two itineraries: a four-day loop north to Lizard Island and a three-day loop south to Hinchinbrook Island. Guests can do either or, as I did, run them together for a seven-day figure-of-eight route.
It was mid-winter, which meant the water was 24C and you could stay in forever. It was a bit cloudy and windy that first day, but the coral and fish don't care about that, so there'd been plenty of takers for the snorkelling as well as the glass-bottomed boat, which seated 20 and launched from a forklift-style cradle at the back of the ship.