Interactions with boats can cause several behavioural changes in cetaceans. The purpose of this research is to analyse if Guiana dolphins, Sotalia guianensis, change their surfacing patterns in the presence of different boat categories, and their contact distances to boats at Port of Malhado, Ilheus, Bahia, Brazil. Data were collected from a fixed point from September 2008 to August 2009 and totalled 362.6 h of sampling effort and 213.22 h of effective effort. The number of dolphin breathing events was recorded during one minute periods, every time a boat passed nearby, and the same measurements were performed during periods of boat absence (control). Dolphin group composition was classified into groups with calves and groups without calves. Boat types were classified as inboard motor (IM), outboard motor (OM), ships (S) and without a motor (WM). Distances between the dolphins and passing boats were classified as near, intermediate and far. In total, 365 samples of one minute observations in absence of boats, and 379 observations in their presence were collected. Inboard motorboats had the highest occurrence in the study area (n = 478) and in interactions with Guiana dolphins (n = 260). However, outboard motorboats were mainly responsible for the reduction in surface patterns with an average of 1.82 and median of 2.2. In groups with calves, the number of breaths decreased significantly with an average of 2.13 per minute. There were no significant variations concerning the distances in the breathing pattern for any of the boat categories. The variation in the S. guianensis breathing pattern in the presence of boats may be a strategy of boat avoidance or a response to the difficulty of communication between animals.