I had a 1999 185 Sport that had a problem with the sending unit when I bought it. I got them to repair it under warranty, but you are correct, (and I could not believe it, either) there is no access to the fuel tank! They had to cut or drill an access hole (in to my brand new boat)to get at the sending unit. They did this right where the back pedestal seat receptacle was and then installed a rectangular stainless steel plate with a new seat receptacle built in to it over that hole. It actually looked fine and held up for the 9+ years that I had the boat, and you couldn't tell that it didn't come from the factory that way unless you'd seen one before. I do not know if the dealer I took it to fabricated this from scratch or whether it was a tech bulletin or something from Tracker. I couldn't get any reliable information from anyone, but was just happy that it got taken care of. I think this problem with my boat is what originally led me to this board.
I had to sell that boat, which my family and I loved but didn't get to use as much as we wanted, when my daughter reached driving age. Had to sell that beautiful, sparkly, red-hulled, fish-catching, tube-pulling, skier-towing, cruising-around-at-sundown, 1999 Nitro 185 SPORT so we could buy an ugly, old, rusted, dented, blue-green, leaky-headgasket, needs-new-tires, deer-smashed-into-the-drivers-side-door ......, etc, etc,....short-in-the-wiring-to-the-fuel pump,..etc, crappy, old, 1995 Dodge Neon!