The full-day lagoon cruise is the thing most people come to Aitutaki to do, and it delivers. A boat — typically holding a dozen or so passengers — spends the day working its way around the lagoon, stopping at snorkelling spots with abundant coral and fish, pulling up to motu beaches, and eventually delivering everyone to One Foot Island for a couple of hours before heading home. Lunch is a BBQ cooked on a sandbank: grilled fish, chicken, salads, and enough food that you'll be glad you swam beforehand.
Several operators run versions of this tour and the basic shape is similar across them: morning departure from Arutanga or the main wharf, snorkelling, island stops, BBQ lunch, One Foot Island, afternoon return. The differences lie in group size, the boat itself, and how much the guide knows about the lagoon's ecology and history. Smaller operators often offer a more personal experience; the larger tour companies are reliable but less intimate. Book directly through your accommodation if possible — they'll know which operators are running well that season. Bring sunscreen, a hat, and more water than you think you need. A full day on the lagoon, even with cloud cover, is full sun exposure.