Ecotourism boats could indeed be harming dolphins and whales, for example by interrupting their foraging behaviour (see Nature 512, 358; 2014). But many whale-watchers do right by the animals and follow good practice. The major threats to cetaceans are still hunting, fishing, military sonar, undersea explosives and pollution.
According to the International Whaling Commission, there were just 32 collisions with ecotourism boats, resulting in 4 whale fatalities, between 1885 and 2010 (see go.nature.com/vehx93). Private boaters who do not follow proper guidelines are a problem. And ecotourism boats frighten Icelandic minke whales (Balaenoptera acutorostrata) probably because the animals confuse them with Iceland's commercial whale-hunting vessels.
Even where ecotourism boats have damaged cetacean populations — as in New Zealand's Doubtful Sound, where tours caused bottlenose-dolphin numbers to fall by 11 between 1997 and 2005 — these declines pale in comparison to the hundreds of pilot whales that are butchered each year in the Faroe Islands, or to the similar numbers of dolphins slaughtered in Taiji, Japan. Moreover, some 300,000 cetaceans die every year as a result of entanglement in fishing gear (see go.nature.com/bh2wl7).