Oceana 2008 declared that Tourists are willing to pay more for a dive trip during which they are likely to see marine wildlife; including approximately $55 to see a healthy coral reef.
Elham, in one side, the visit frequency and the density of diving groups can affect the health of the coral reef, but on the other side, the resources payed for the tourist could be used in the coral reef conservation.
Elham, my student just finished work looking at tourism on grey reef shark behaviors (see today's Coral Reefs for a preliminary reef site article about this work). Tourism can certainly impact on the health of a reef (anchoring, pollution, impacting animal behaviors etc) but without interest in the reef environment (or the tourist $$$ it brings in) coral reefs may not be able to be conservatively managed. As mentioned above its always going to be a trade-off
I have supervised some undergrad students whom had to do thesis regarding this issue. have attached the files here. One was sponsored by the Swedish International Development Agency and the other was presented at a small conference in Osaka, Japan
In the middle of the Sulu Sea in the Philippines is a national park called the Tubbataha Reefs, that is a highly-managed in terms of number of visitors & allowed practices. The motivation driving the management is the willingness to pay by the visitors to have an extraordinary reef diversity experience.