Well... depends on the boat a bit but to get to it in my 1995 Sprint, unscrew the hinge from the rear deck hatch and hang upside down with your head in the bilge looking forward.
Pick the nearest hatch that doesn't have a tub installed that you have to remove the upper deck to access. You might get lucky and be able to get to the fitting. Or you may end up having to cut a hole in a tub... or install an access port in the deck or under the seats...
They do some of the plumbing before they join the "deck" to the hull...
Post some pictures so we can see what you are dealing with. Pictures always gets better ideas and suggestions. I do like the cutting an access panel, but until we can see the situation, it is hard to make other suggestions. JMHO!
