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Dolphin watching is a relatively newly developed tourist activity. Although it is assumed to contribute to the conservation of marine ecosystems, its long-term adverse effects are still unknown. The present work aims to assess the impact of tour vessels on the behavior pattern of dusky dolphins displaying diurnal and cooperative feeding. We evaluated potential bias of commercial vessels to specific activities, short-term responses of dolphins to vessel approach, and longer-term effects of these changes, particularly on the activity budget and the time dolphins need to resume an activity. Several field experiments were carried out in which dolphins were approached by a research vessel or by commercial vessels. Group tracking was also made from a research vessel. Behavioral sequences were modeled by stochastic matrix models. Activity budget and time to return to an activity were obtained from matrix properties. Commercial trips found mostly feeding groups. Feeding showed the highest rate of change at the moment of vessel approach, and commercial vessels had an effect on the direction of change. Over the longer term, feeding and socializing time budgets decreased when commercial vessels were present. The time to return to feeding as well as the time it took for a group of dolphins to feed increased in the presence of commercial vessels. Dusky dolphins in South Atlantic waters feed during the day on patchily distributed prey. Our study demonstrates that boat presence negatively affects the longer-term feeding efficiency of dusky dolphins.