Hard pipe is ok for long straight runs, but
those are hard to find on most sailboats. Plus, it should be "soft coupled" to everything rigid--toilet, tank, y-valve, macerator, thru-hull etc--with about a foot of hose to provide shock absorption and protection from flexing. All that means a LOT of unions that can all be potential leakers and traps for solids and TP. There a people here who've plumbed their whole systems with hard pipe and claim to have no problems. If you're on a lake or a bay where seas never get worse than 3-5'--and never go out when it's even close to that, you prob'ly won't either.So I wouldn't do it...but it's your boat, your choice.