The present work uses daily checklist data from whale-watching to study basic aspects of cetacean ecology. Data in the Azores consists of a simple daily list of all species sighted by boats, with no additional information. The Azorean whale-watching activity is described and sources of variation related to data collection are debated. A data-set from one whale-watching company is analysed from 2001 to 2006, from April to October (tourism season). The first four years were used to describe frequencies in the area, and maximum and minimum regional sea surface temperatures for nineteen cetacean species, based on remote sensed data. Several species were present throughout the season, while for others, results indicate transient habits in the study area (Balaenopterids, Globicephala sp., Pseudorca crassidens, Orcinus orca and Hyperodoon ampullatus). Reservations were made relative to small size and evasive species. Sighting frequencies on temperature classes in 2005 and 2006 were used successfully in an experimental approach to study delphinid simpatry. The simplicity of data was overcome by the diversity of sighted species and the dimension of the time scale. Daily checklist is a low cost useful source of information mainly for large cetaceans that should keep being registered.