I will check into which material is used for the water tank - luckily it is in a position where it can easily be replaced if necessary.
Because the fuel pickups & return tubing is complex I wanted to keep it simple and just use one tank, but after reading the replies I think I will put in 2 separate systems. I think I'll go the route of a new tank versus a bladder, though - I have this image of the bladder chafing and me having 250l of diesel in the bilge.
Omatako - the fuel consumption of 15l/hour is at max cruising speed (i.e. what I'd use to get out of the way of a big depression) and is the published figure by Tohatsu for 2800RPM. I get good mileage at 2000rpm as well and fuel comsumption is in line with 5l - even better when motorsailing with the Gori prop set to "overdrive"; but I would like to be able to have the option of running more than 24hours at "flank speed" - naturally in the hopes that I'll never actually have to do it. As mentioned before, I don't motor much and most of my fuel is consumed running my genset, but for the big pacific distances I would prefer to have reserves.
(addendum) Yanmar 4Jh$ fuel consumption