Concierge.com's insider take:
Forget everything you know about swimming-pool "dolphin encounters"—nothing beats meeting these creatures in the wild. The channels and coves that make up the Marlborough Sounds, on the island's northeastern tip, are home to masses of dolphins—including bottlenoses, Dusky dolphins, and rare Hector's dolphins. Dolphin Watch, based out of the port town of Picton, makes daily trips on 30- and 40-foot motorboats into Queen Charlotte Sound. Trips last between two and four hours; passengers scout and follow dolphin pods, and if conditions are right, slip on wet suits and snorkels and jump into the water with them. The dolphins are fearless and playful, sweeping within a few inches of swimmers; it's completely magical. If dolphins can't be found or the weather's too rough for swimming, the tour company gives free vouchers for another trip (64-3-573-8040; www.dolphinswimming.co.nz).