Safari tents could be really cool as long as the weather is nice. In Portugal that’s probably not much of a problem. It would definitely be unique.
The only time that I have seen safari tents was on a safari in Kenya. It wasn’t a hostel, but it could have been without changing much. Sadly my photos of the place are in a box on the wrong side of the Atlantic, but here’s a link to the camp with a few pictures. Each tent had two rooms divided by a canvas wall: one was for sleeping and the other was a full bathroom with sink, toilet, and shower.
If you have a main building in addition to the tents it would be great to have safe storage for the surfboards and maybe even a workshop where your guests could do repairs.