It's best to change out the hoses. Yes, you're correct; it is calcium carbonate. It forms a gritty, grayish deposit that sets up like concrete. You can use standard muriatic acid (30-35% HCl) mixed in the ration of 1 pint muriatic acid to 2 gals. clean water. Mix it in a plastic bucket, then pour the mixture into the toilet bowl. Muriatic acid can be bought at hardware stores and home improvement centers. It is used by cement workers to clean ceramic tile, terrazzo, etc. Pour the solution into the toilet bowl and flush it until it goes into the lines. If the toilet flushes directly overboard, station someone at the seacock for the toilet's discharge. As soon as the acid solution has left the toilet bowl, yell for the other person to turn off the seacock. Leave it in the off positon for two hours, then open it and flush the toilet normally for at least a dozen times, to clear the lines.
If you're flushing to a holding tank, you'll have to add an inline shutoff valve near the entry to the holding tank, or find some other means to hold the solution in the lines for an extended period (1-2 hours) so that it has the necessary time inside the line to dissolve the calcium deposits. Then again, flush the toilet for a dozen times, to clear the lines.
To prevent future buildup in the lines, pour a cup (8 oz.) of white vinegar into the toilet bowl and flush it away, once a week - usually on Sunday night, before leaving the boat. White vinegar will also dissolve the mineral buildup, just as it will dissolve egg shell (same mineral composition), but it works much more slowly. In that quantity (8 oz./wk), it won't affect the contents inside the holding tank or kill any of the necessary bacteria inside the tank. But keep it to a maximum of 8 oz./week - no more!
Note: muriatic acid is dangerous to use - be sure to use eye protection and observe (and follow) all warnings on the container.
