One fish, two fish, 72 fish

Two-year-olds are notoriously bad snorkellers, which is hardly surprising. Having barely mastered the fine motor skills needed to skewer a pea with a fork, they could hardly be expected to come to grips with the tricky business of swimming while wearing a mask, fins and floaties.

This is a shame because they would probably be very handy with a harpoon, assuming their prey was small, green, round and didn't move too quickly.

The snorkelling off Heron Island, at the southern end of the Great Barrier Reef, is superb but, like all snorkelling, it's not a young man's game: it's something for the over-three crowd.

The glass-bottomed boat tour appeared a much better option: two hours on a semi-submersible, sitting in comfort and safety, admiring some of the world's most amazing ocean life through glass walls. That was more the kind of thing that would suit us.

Sure enough, the two-year-old was impressed from the start. "Fishy! Fishy! Fishy!" he shouted, every time he saw one. This was, of course, incredibly cute and endeared us to the rest of the passengers, who smiled, laughed and shook their heads in amazement at what a splendid, intelligent child we had.

However, by "Fishy!" number 72, I got the impression the novelty was beginning to wear thin.

The signs were subtle but noticeable: passengers refusing to make eye contact, those nearest to us sticking a finger in one ear and the marine biologist raising her voice pointedly as she explained how certain fish eat their own offspring.

This seemed an unusual observation to make, as she was midway through describing the life cycle of the lesser-spotted rhubarb clam, or some such creature, which wasn't a fish at all.