Google Ads simply don't work - I tried them at the beginning, for a year or two, but the revenue was between $1-$4 per month. Google simply marks most of the click-throughs as invalid, without any explanation. Besides it, their script crashes often and slows down the website terribly. And additionaly the content of their ads is often rather irrelevant, and it is quite difficult to control it. Some of their ads are quite annoying. All in all, the negatives overweighted by many orders the very little revenue it generated, so I gladly removed it, and repalced with the free advertisement, that I can control, and that instead of annoying my visitors, adds more content to it.
Most diving centers are oddly not even interested by the free advertisement - it comes with access to statistics, where they can see the numbers of banner views, and click-throughs, and they'd have an easy way to bid for higher exposure. But I admit, I spend little time with marketing it personally. For now I prefer adding functions, data, and other content.