Many of the Dessalator answers can be found at http://www.dessalator.com/ including your size and some instructions on replacement. I see you found the correct membrane. For all of you that do not know this, there is currently 1 manufacturer of these membranes in the world: Filmtec. Filmtec membranes will be found under other brand names like Spectra, but in the case of Spectra, the membrane is a special size and can only be bought at Spectra...this is why the Spectra membrane is $600. 

http://groups.yahoo.com/neo/groups/amelyachtowners/photos/albums/1219017177

Regarding the Water Heater, I do not believe the internal water pipe can be replaced. I know the electrical heating element can be. But even the electrical will be 30% of the cost of a new heater. I am fairly sure that you want to go with a new heater because removing the water heater is a pain and surely if you replaced one thing today, something else will go wrong tomorrow. Isotemp makes the exact model number of the 40 liter Amel installed in #387. They also make the same water heater with two water paths i.e. can heat the water with two engines, or cap off the 2nd path for use later. If I remember correctly, this was only about 10% more, but a special order item. If I had time, I would have replaced the original with this and plumbed it to the generator and the main engine. I did have some issues with the fit of the "exact model number" because Isotemp changed the model, but not the model number...see the following link for that information:

Hope this helps.

Bill

BeBe, #387