When I bought my 1987 Maramu, the water had been sitting in the tank about 1/3 full for 4 years.  I did not drink it as it smelled a bit “swampy”.  Upon opening the tanks, I found snails and a considerable growth of black mildew, mainly on the upper portion and undersides of the tank.  After a thorough scrubbing and removal of the organics plus flushing the water tank a few times, the water no longer has any smell or taste.  The interior coating which appears to be gelcoat on my boat is  is cracked in a few places and  so the water is getting to the laminate in places.  There were also a lot of gelcoat chips in the bottom of the tank which were getting into the plumbing and plugging the faucets.  The interior of my tank showed no blistering at all but the exterior of the hull which has never been barrier coated also shows no blisters (and no voids when tested with Ultrasonics)  so this may be a function of the resin used.  When my boat is laid up, I completely drain the tank, open the very fwd and aft access port and run a fan for a couple of days to remove the rest of the moisture.  (I was told by Michel Charpentier that this was the best way to leave the tank if the boat will be in storage for a while and it makes sense to me as well) This should reduce the future mildew growth.   While the tank water now tastes fine, I have been using bottled water for drinking where possible for personal consumption unless I am out of the bottled water.   My long term plan so currently is to recoat the interior of the tank with a safe coating and to then install a low pressure RO unit such as is used in homes (driven by the city water pressure) for the drinking water source.  

   The 1000L water capacity is a great feature of the boat IMO.

James Alton

SV Sueno,  Maramu #220 1987