Morning beats afternoon, most days
If you only take one thing from this page: book the morning.
Wind is the reason. Hawai‘i’s trade winds typically build through the day, and wind means surface chop. Chop scatters light, stirs sediment and makes the surface harder to look through — so visibility is usually at its best early, before the trades fill in. Calmer water is also easier for nervous or inexperienced snorkellers, and gentler if you are prone to motion sickness.
Light helps too. Higher sun puts more light onto the reef and brings out colour that reads as washed-out and grey in flat early-evening light.
Our Turtle Canyon trips run at 9:00am and 12:00pm. The 9:00am is the better of the two on most days — and it is also the one that books out first.
What the seasons actually change
Water temperature barely moves. Hawaiian coastal water runs roughly in the high 70s°F in winter to the low 80s°F in late summer. Nobody needs a wetsuit, in any month.
Surf is the real seasonal difference, and the good news for Waikiki is that it works in your favour. The big winter swells O‘ahu is famous for hit the North Shore. Waikiki faces south, so its bigger surf arrives with southern swells in summer — and even then it is modest compared with what the north side gets in January.
Winter brings humpback whales. They are in Hawaiian waters roughly December through April, and we see them from the boat on winter trips. They are not approached deliberately — federal rules require keeping a distance — but a humpback surfacing on the sail back is a better encounter than most people get on a dedicated whale tour.
Things that matter more than season
Rain the night before. Heavy runoff puts sediment in the water and drops visibility for a day or so. Turtle Canyon is far enough offshore to be less affected than beach-entry spots, but it is not immune.
Where you are in your trip. Do not book a snorkel for the morning you land. Jet lag, dehydration and a tight schedule make for a bad first time in the water. Equally, do not leave it to your last day — if conditions cancel it, you have no second chance.
Whether you have booked. The sunset trips in particular sell out, and the 9:00am snorkel goes early in high season. Availability is live on the booking calendar for each trip.
So which do I book?
To see turtles and get in the water: the Turtle Canyon snorkel, 9:00am departure.
To be on the water without getting in it: the Afternoon Sail (April–November) or the Sunset Cruise (year round). Both include the open bar.
If you are in Waikiki long enough, the snorkel and a sunset are a genuinely different pair of experiences rather than two versions of the same one.