Additionally, now having gone full-screen, I see a filler cap in front of the top cross piece, plus an outlet tube in the bottom right hand corner.(Partially hidden by the ski itself.) The main fuel tanks would appear to be in the outriggers: Note the lines down the rear edges, accessed via the two round covers on the after ends. The auxiliary tank for the jet ski looks like an after-thought I must confess.
(Probably two-stroke fuel.)

Well, there is a small hand-winch attached, for pulling back on-board the SEA-DOO/JetSki....so maybe it's a fuel tank for replenishing that?

What is that metal box on the stern deck right behind the door? why not just leave is as deck space. they need as much as they can get.