Muriatic acid works
A 12% solution in water. You can get it at any hardware store. Be very careful handling it...read and FOLLOW all directions.That "rocklike substance" is a buildup of sea water calcium carbonates--or, if you flush with fresh water, hard water minerals. You can prevent it from building up in the future by flushing a cupful of undiluted white vinegar through the hoses once a week or any time the boat will sit. In fact, white vinegar will also dissolve the buildup...but will take quite a few applications to dissolve enough of it to completely clog a 1.5" hose...muriatic acid is faster.Or, you could do what a lot of liveaboards on very tight budgets do: remove the hose and bang it on the dock to knock all the minerals out, then put it back. Seems to me, though, that if you're gonna do that, you might as well put NEW hose back.I'd be cautious about using something used by cruise ships...the systems on 'em are the same as hotels and other commercial buildings...they don't include hoses. What's safe to use on land may not be safe to use in sanitation hoses.