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Coastal cetaceans are increasingly being exposed to boats and noise as nature tourism grows. Such activity has a wide range of detrimental effects on the surface behaviour of cetaceans, but effects on their acoustic behaviour are poorly understood. We quantified the effects of tour boats and of the observing research boat on the group structure and vocal behaviour of bottlenose dolphins Tursiops truncatus in Doubtful Sound, New Zealand. Acoustic recordings and group follow data were collected from a 5 m research vessel, and analysed via an information-theoretic approach. Groups with mother-calf pairs were significantly less cohesive and coordinated when tour boats were audible. They were more vocal when boats were close and while moving away, presumably to re-establish group structure. Furthermore, groups with calves increased their whistle rates when tour boats were travelling faster, while groups without calves became quieter. Dolphins also responded to boat noise with alterations in whistle frequency and duration. These findings suggest that elevated boat noise affects communication, and groups with calves are particularly sensitive to boat presence and noise. Group structure and whistle parameters were affected by the research boat, highlighting the importance of accounting for observer effects in studies of tourism impacts. The particular sensitivity of groups with calves to boats has important implications for the management of impacts on this population due to its endangered status and history of low calf survival.