Ah, OK. I understand, now.
Without being there, looking at the boat with you, I can't say where exactly the access to the pump is - but it is usually close to the tank. It may awkward to access (things usually are in boats), but you shouldn't have to cut anything to get to it. See what is "removable" in that area... A removable portion of a seat, for example? Maybe post a few pics of the area where the tank/pump is located?
Have you verified power getting to the pump once you turn it on? Checked breaker/fuse?
You have visually confirmed that there is water in the tank?
Not all tanks have a drain - many do not. Often, the tanks have top mount fittings only... meaning the dip tube would be the "drain". If the pump isn't working, disconnect the input line at the pump (the line that comes from the tank) and use whatever type of pump you have laying around (had pump is fine) and pump/siphon the water out. It's OK if an inch or two of water remains - that will NOT cause any issues if/when it freezes. Next step would be to use that aforementioned pump and connect it to the output line. Pump non-tox AF through the systems that way. A cheap hand pump will do the job.