Rarotonga, Cook Islands

Rarotonga is the main island of the Cook Islands — a volcanic peak rising to 652 metres, ringed by a fringing coral reef and a shallow lagoon that separates the island's shore from the ocean. The reef is almost continuous around the island, broken by passages (avatara) cut by freshwater streams flowing from the central mountains. Those passages are the arteries of the lagoon: tidal water enters and exits through them, and the state of the tide determines whether the lagoon is calm and snorkeable or opaque with suspended sand stirred by swell wrapping through the cuts. The tidal range at Rarotonga is modest: mixed semidiurnal, spring range 0.5 to 0.8 metres. Two unequal highs and two unequal lows per day. By global standards that range is small — the lagoon does not drain or fill dramatically. But the lagoon itself is shallow: 0.5 to 2.0 metres over most of the sandy inner floor. A 0.6-metre tidal movement over that depth changes the character of snorkelling conditions measurably. At high tide the inner lagoon has enough depth to snorkel comfortably over bommies; at low tide on the neap, the sand flat on the inner lagoon shelf dries to ankle depth in places. The ocean side versus lagoon side distinction matters more than the tidal range in isolation. Rarotonga's windward (southeast) coast faces prevailing trade-wind swell directly; the passages on that side are surged even at high tide when the trades are up. The leeward northwest coast — from Arorangi to Black Rock — has a calmer lagoon environment and is the favoured paddling and family swimming zone. Muri Beach on the southeast lagoon is a partial exception: it sits behind an effective section of reef and four offshore motus that diffract swell, making it the most sheltered and popular lagoon spot on the exposed coast. Tide data for Rarotonga locations comes from Open-Meteo Marine. Accuracy is typically ±45 minutes on timing and ±0.2 to 0.3 metres on height. For the shallow lagoon environment, the height accuracy matters: a 0.2-metre model error on a predicted 0.4-metre low represents a 50 percent uncertainty in available depth.