I have an Iwata - it is a top load with a 2cc cup and I use plain old rubbing alcohol for cleaning water based paints, works like a charm!
I do a full cleaning after each use, dump some alchohol (90%) in the reservoir and spray it trough, you can do 50/50 water alcohol too, works just as well. DO NOT BREATH THE VAPOR, and use a well ventilated area! It stinks and will BURN if you breath it in, especially when you mix it with water it seams more potent.
I take apart the tip and put that in a little dish of alcohol, then I take the needle out the back and clean it off very gently, be very careful with the iwata needles, they are very delicate and can bend easy, you never want to use the tip to clean the gun, an old pro told me dropping them can cause damage.
You will want tooth picks and cotton swabs to aid in your cleaning. I put a bit of alcohol on a cotton swab and run it around inside the reservoir. Then I use a tooth pick to get the stubborn bits. After that I reasemble the needle and tip, run more alcohol through the gun, and if the reservoir is clear and clean, I do one last run and call it good.
Cleaning it takes a good 10 minutes but it really is worth it to not have to clean the gun before each use.
If you have lots of paint build up, you can take the gun apart and leave it in a bowl or tupperware container over night soaking in alcohol, it wont hurt a thing. Then clean the empty gun body under running water the next day, inspect all parts for paint and rub the paint off any parts with a tooth pick or cotton swab, then reassemble and you should be good to go.