Well that's buggered it
Assuming you don't want to take the rather drastic action of selling the boat and buying
another, all the toilets can be taken apart.
You haven't said which model you have, but when I did ours (blocked waste pipe) I sent wifey ashore for the day with the credit card. Wore only a pair of shorts and set to work dismantling the heads. Actually it wasn't as bad as I expected. If its the Jabasco type of heads replace the joker valve whist your at it because they are cheap
and fail eventually.
Any pipes that appear encrusted with a concrete material on the inside need to be taken ashore and beaten against a wall to remove said build up.
As for the knife well tie to a piece of string, chuck over the side and go sailing.