Studies have found that marine tourists can cause impact to the underwater environment by breaking corals and disturbing coralassociated benthic organisms. However, which type of dive tourists inflicts the most harm on the marine ecosystem? Few studies are able to throw light into this.Making use of data from the direct observation of diving behaviour and self-reported findings on the environmental attitudes and values of 80 Hong Kong divers, this chapter brings to light factors or attributes that would affect or associate with divers, underwater behaviour. It was found that by carrying a camera, divers made 9.8 more intended contacts per dive.Divers who are not so willing to avoid using non-biodegradable shampoos (a pro-environment behaviour) would also make more contacts on the marine biota. However, even the more disciplined divers in the sample are not so willing to financially contribute to marine conservation.On the whole, attitudinal and value-based variables can only explain about 20% of the variations in their underwater behaviour. Given existing findings, it is advisable to the underwater tourism industry to not just educate but also impose dive guide intervention to reduce divers" intended contacts on marine biota to protect the already stressful underwater environment.