How do I add water and diesel sensor?

[Jodel]

What about the following; This is what I'm planning to use for my water tank.:

http://www.vegetronix.com/Products/AquaPlumb/

Looks good. 
Low current consumption 1.2mA !
I'd be interested to hear how you get on with it.

I notice that it says that it works in oil also.  I presume that means it could be used to check the level in a diesel tank.  The wire would have to be fixed to something to ensure that it was not swinging around in the tank as the boat moved.
Jodel