I use these:
http://www.duckclean.com/freshdiscs/uk/ (Lidl sells them). Comes with a plastic applicator and just sticks to the side of the bowl, no wonky plastic hangers that fall off. They last a good long while and seem to prevent limescale buildup and keep the bowl clean, although I'm not sure by what mechanism - they don't seem to contain acid. Ingredients may not be too environmentally friendly
, but it releases only a very small amount per flush. No buildup in the toilet or pipes yet, but it hasn't been too long since I serviced the whole contraption. Nota bene, I have plastic seacocks, so do not need to worry about anything attacking the metal, although with no acid on the ingredients that is probably not a concern.
Edit: I've also read and followed the Jabsco instructions that tell you how often you must pump per meter pipe to clear the whole length, and for my boat that works out to 15 pumps for a complete flush of the pipe, which I do (and crew just accepts this as part of weird boat toilet rituals).