The best method is to monitor the are with under-water mounted cameras.

I believe your question is very broad. You may need to define your goal or what impact you are considering, such as population, fish behavior, fish health etc. Then you can find a proper methodology.

There are a variety of methods to assess fish communities on reefs. Traditionally a number of visual census techniques have been employed with the most frequently used methods including line transect count, stationary point count or timed swim methods. Generally these surveys, regardless of the method used, aim to characterize and quantify the composition of the fish assemblage (i.e., abundance, size structure, and species composition). If you want to assess recreational divers on a reef area however your sampling design becomes more difficult, because ideally you need replicated reefs that are ecologically similar, have the same pressures (e.g. areas with no fishing or the same levels of fishing) and only differ with respect to the presence or absence of  "recreational divers". Another alternative or complementary option is to deploy underwater cameras, as suggested by Karbassi.  Data from underwater cameras has been found to provide good measures of fish assemblages. This is because the underwater camera method avoids biases caused due to the fact that some commonly fished species are not recorded well by visual census diver survey methods. Underwater-cameras will also enable you to assess changes in fish behaviour with the presence of 'recreational divers' in the area. Good luck with your research.