TB, you may not be using your shower much and so don''t notice the consequences of putting that nice mix of hair, soap, body fuzz and other biologics in your bilge, where a portion of it sits, breeds and begins to stink, given that almost every bilge fails to totally empty. The Columbia 30 owner was, I believe, thinking of more frequent use. In such cases, it''s worth the effort to install one of the integral shower sumps or put one together on your own (mini-float switch, sump body and in/out tubing run). It will seal in odors, be more accessible for cleaning, and is a much smaller cleaning job.
Plumbing shower grey water into the head
holding tank is...well, let''s just say a ''science experiment'' in multiple disciplines.<g>
Jack