Tell us what brand of boat it is, etc. AD's assessment is spot on. Fuel tanks are installed into the hull before the 2 boat halves are put together, so big stuff like the fuel tank(s) don't come out easily.
Post some pictures of the tank and any deck hatches or sections that could be used as access.
AD and I both have threads that show fuel tank removal His is a Formula 27 and mine is a Formula 330. Bigdirty did a tank on a Crownline 250 a while ago. You can use the search feature to find the threads.
