Be careful about addressing the surfing directly. You aren't going to make your employees happier by tightening the leash.
If your employees are delivering the results you expect and surfing, where is the problem? Let them be or increase their workload. If they are not delivering results you are happy with then you need to address those results and not necessarily the surfing.
If you are concerned about criminal activity, then you can inform the employees that you will be checking the logs regularly and that things such as pornography will not be tolerated.