Go over to sailboatowners and look in their store for 
Peggy Hall’s book on boat odors. It’s great.  

The shower should use a common drain with the sink.  The sink/shower drain are right next to each other so maybe he got confused.  The head inlet should have a small filter to prevent big stuff form plugging up the pump.  Note bothe of these are “raw” water not connected to your potable supply.  If your getting stinky water from the sink you may need to shock the system. 

There should be no odors in your boat boat and if so Peggy’s book is the Bible for removing them

Les

Go over to sailboatowners and look in their store for 
Peggy Hall’s book on boat odors. It’s great.  

The shower should use a common drain with the sink.  The sink/shower drain are right next to each other so maybe he got confused.  The head inlet should have a small filter to prevent big stuff form plugging up the pump.  Note bothe of these are “raw” water not connected to your potable supply.  If your getting stinky water from the sink you may need to shock the system. 

There should be no odors in your boat boat and if so Peggy’s book is the Bible for removing them

Les

Mark,

I had the same problem when I purchased Slip Away and it stinks!  Look in your manual or google how to sanitize your freshwater tanks.  Basically fill your tanks and add bleach/chlorine, let sit, drain and rinse.  Then as you fill periodically throughout the season add a capfull to keep it fresh.  No more smell for us!  Make sure to run hot water side so as to also circulate through heater.  Not sure how 2001 shower drains but our 1996 drains into sump in floor which has its own bilge pump and is pumped overboard via a switch for pump.  Rinse and clean, rinse and clean, rinse and clean...... you get the picture.  Our drains had a smell to them,  rinse and clean,  close through hull valves, add bleach and let sit, rinse and clean.  

On my new to me 1995 Catalina 36 Mk II the fresh water smelled like a swamp.  One tank fill is missing its correct cap, what was there was just held by gravity.  
All tanks will start to 'go bad' if left stagnant for long.  Make sure your deck fills seal correctly, and as others have suggested shock your tanks.  You may even want to get inside them and clean them in case they have any growth inside.  
 

I decided I wanted nice clean tanks on my 2001 and so I unscrewed all the tops from the 3 tanks, and was very creative about getting all the tanks clean using nice long handled brushes.  I was truly amazed at just how much gunk was in the bottoms.  They had a nice layer of mud and muck in the bottoms.  If you have never cleaned the tanks out, I highly recommend it.  All the clorox and cleansers in the world won't get rid of stuff in the tank. I would almost guarantee there is stuff in the tanks if they haven't been cleaned.

When the boat is on the hard, I unscrew the tank lids and blast the inside with the hose nozzle. Bit of a tight fit, but my hand and hose nozzle can access the inside. I then fill each tank with a couple gallons and drain. Then I add another couple of gallons with a high concentration of bleach and drain again. When I drain (with use of the fresh water pump), I remove the spray fittings from the galley, cockpit shower and head shower so if there's debris in the tanks/line, it wont clog the spray heads. The first time I did this I also removed the screen from the fresh water pump, as it accumulated a lot of debris. This also removes the 1-2 gallons of antifreeze/tank from winter storage. 

Our boat is relatively lightly used, so I generally only half fill the tanks and add chlorine, and that's enough to take us through the summer. We are on a mooring and  if we run out of water, I fill a couple of 5 gallon containers, add chlorine, and set them on the stern seats. We  empty into the aft water tanks via syphoning- which is easier than pouring. We use water for cleaning and flushing, not drinking. Fresh water head flushes via filling the bowl from the head hose helps to keep holding tank stink down. We generally use the hot water tank  for flushing to ensure it doesn't sit too long in the water heater tank.

Also, for hot water tank winterization, I disconnect the hoses and drain into the bilge. When it's "empty", I use our inflatable foot pump to blow out any remaining water. I use a couple feet of bypass hose to connect the disconnected tank hoses. This eliminates the need to winterize the tank with antifreeze. 

Leslie Troyer is a treasured and reliable 'old salt' and I have to concur with his post. Your experience with both shower and fresh water both being smelly is a coincidence related to lack of use rather than a direct connection in the plumbing. As for the other comments, yes, even the most pure tap water will go green over time in a tank and will need a strong bleach treatment, fill to the very top of the tank, let it sit for 15 mins, then run the tanks empty through the galley and head sinks, both hot and cold, then flush with another fill of clean tap water.  

At our club being ten slips down the dock with a lot of members rarely using the tap water, we have to run the water when we arrive on the weekend for several minutes with our nose in the sink to check until the skunky smell is gone.  I've used the tank freshener products a few times, but a capful of bleach likely does the trick to kill the potential for odors. 

That being said, our boat is now 34 years young and I've had the tank access caps off, but there was really nothing inside, no build-up of any kind (maybe the two PO's were also super fastidious about keeping the tanks either empty and/or clean).  I have the vessel outfitted with city water intake fittings (Jabsco) both at the bow and at the stern (they have built-in backflow preventers and pressure reducers/controls), as well as a system for filling the two tanks we have from under the galley sink with two 90 degree valves, so I rarely have to use the stern tank-fill fittings.  We run the center tank dry regularly, then switch over to the starboard tank and empty that before refilling, so they stay clean. 

Over the past few years I've stopped putting antifreeze in and just empty both tank and leave the valves open, then use a mini-shop-vac to suck the lines dry pre-winter.  The hot water tank had a LOT of residual water in it, so it does take either pressure from a 12 volt air pump to remove the water, or a lot of vacuum tank-fulls to empty it completely.  Similarly the lines to the head and the shower drain set-up you have might need some special attention.  For the latter I installed two one-way valves in series to ensure that the residual smelly water in the outlet tube (after the sump pump) does not come back into the shower well.  Took me a few years of fiddling and pondering to get to an odor-free environment!  (Adding a "BIG ORANGE" charcoal filter with a anti-suction vent loop above the black-water tank eliminated the flushing stink-cloud from choking out guests in the cockpit...)