Ok, Ive asked this sorta question in the past, and heres the response Ive gotten:
Dont use drains with large monitors- the mass amount of feces (especially from something like a large Nile) will EASILY clog any sort of draining system you install.
This leaves you with 2 general options.
1. You can go the simple route, which in and of itself has 2 ways to go. Both are basically just using a large water container(generally at least large enough for the animal to submerge/swim in) that can be easily removed for cleaning. These are the 2 basic ideas:
A. You can just use a large feature and position your cage so that its easy to remove and dump/clean.(this greatly depends on where you keep your animal in the home- if you have it in your bedroom, this may not work as well as if you have it, say, in your basement)
B. You can use a large feature and, using a siphon pump (pretty cheap at most auto-stores; got mine at pep-boys), remove most of the water without drawing in too much feces.
(Note: The problem with this that Ive found, is that over time SOME particles of feces and substrate DO manage to make their way into your siphon, which makes it nearly useless if you dont clean thoroughly after each use)
2. You can go the expensive route, and install a pump with a backflow that can pump out any sludge/poo that gets clogged in the drain- all the same, im told it still results in constant maintenance to keep working.
My suggestion?
Either use a singular large water feature that can be removed easily or, what I do, use a relatively small water feature (only JUST big enough for the animal to fit into in case it wants to soak) and offer a humid hide.
Offering an equivalent water feature to an adult nile or other water monitor species is going to result in TONS of mainanence, and constant water changes. While its nice to think of offering some massive setup, if you cant properly maintain it you might as well just forgoe it.
In this case, if you cant keep the water clean (without too much stress to you or the animal), you need to find a new strategy.
Keep in mind- this stuff is NASTY and you want to be VERY careful about handling monitor poop (ALWAYS wash hands, etc.).