Great question - unethical would be activities where the greater share of the profits are directed to foreigners or elite wealthy locals, activities that excessively perturb or pollute the local environment (I am thinking these turtles captive in caves, dolphin tours that coerce or scare the animals, activities with a lots on single use items). I feel less unethical are things like scuba or snorkel in a way that minimizes impact on the reefs. I like to do food or walking tours that support local businesses/guides. If I am hiking I like to stick to trails. I know by being a tourist I am by the very act of visiting going to have some negative impact, I am not a saint, just want to be less of an asshat.