I wouldn't bother with access panels. Typically, the systems only repairs are replacements of the sprinkler heads when they reach an age that the insurance company requires them to be replaced. Drains in the floors are good as long as you don't mind looking at, drains in the floors. Sprinkler heads cover a very area and unless, the floor is sloped to the drain it is not going to help much. As far as the water heater goes you can attach a garden hose and run it to whereever, the floor drain is.
Don't you have enough of rain where you don't need sprinklers?