Jay, Slider, I had a similar fuel leak on my wife's S60 fuel tank cover that looks very similar to the one posted by Slider...
On the S60, the leak was caused by a crack on the hard plastic cover (at the base of the electric connector) and that round piece, is part of the fuel sending unit.
I believe that on the Boxter this round piece supports both, fuel float assy., And the fuel pump assy.,
however, on your picture it seems that the previous owner used sealant as this is not the way it came from the factory as someone already mentioned, there is a large O-ring that seals between the round plastic (yellowish color) cover to the fuel tank.
First you remove (unscrew) the black ring in order to be able to remove the round plastic cover, and then you will find the thick large O-ring.. If the leak comes from a worn O-ring, buy a new one and I would suggest to lubricate it with silicon grease before you install it.
Also, you may want to do this job with an almost empty fuel tank, don't ask me how I learn that..
Note: I believe that the O-ring sits between the round plastic (yellowish) cover and the black locking ring on top. The the round plastic cover sits flush on top of the fuel tank with the O-ring on top (at least on the Volvo..