If you want to make it a model of a real aeroplane, I forsee the same sort of problems that the original designers encountered.
One is to make sure that your trim doesn't alter too dramatically when the load is dropped.
As for the drop mechanism itself, I'd consider just building water-tight bomb-bay doors (clamshell or front-hinged) into the bottom of your tank, which in turn will constitute the bottom of the fusilage. A single servo can open and close them. You might also want to put baffles in the tank to prevent the water from sloshing around too much. I don't know how sensitive models are to shifting loads, but I suspect that they're a lot harder to control than the real thing. (Delayed response time; no sense of "feel"; limited engine power; etc.. Do they normally even have in-flight trim control?)
From the wording of your question, it appears that you want to adapt an existing model rather than build a new one. That rules out the really cool possibility of scooping up water on the fly from a lake or river, but also avoids the highly troublesome aspect of compensating for hydrodynamic drag and ground effect during the loading process.
Those are just my "off the top of my head" thoughts about the matter.
Of course, a lot depends upon what scale this thing is built to.