This data is spatial, so existing maps would be a good place to start. In particular, suitable sources might be:

• Topographic maps - ensure scale is appropriate
• Hydrographic charts - ensure scale is appropriate
• Ariel photographs showing use activities
• Observation / survey - to see where use occurs
• Interview survey with users to discover where they undertake their activity

With any of these data sets it is important that you make sure the accuracy of the spatial component of the data is accurate enough for the study. It is also important to get a temporal fix to appreciate when the data is valid (e.g. swimming is probably only a summer activity). With existing maps there may also be copyright restrictions on their use, so check before using them.