an 18inch water monitor you still wont go wrong feeding daily or even 6 times a week. You got to understand in the wild humidity always fluctuates its not a constant variable, you need to work with your water to see what works best for him start right at the thresshold of say 65% for a couple weeks until his next shed. be aware of his eating patterns how often he spends in his water bowl or at his basking site, is he having trouble pooping. all these are tale tells of how his humidity is affecting him. An 18inch water is still considered a baby in most places so treat it as such. Makes sure you have adequate space, you give him whole foods, a good basking spot, some hide spots for him to fill secure, use substrate that retains adeqaute humidity an also allows the monitor to dig and burrow. Change his water as often as possible almost daily. Notice I did not give you specifics on substrate type, Food or caging, because you can find it on almost any link for caresheet. search these forums there are plenty of links you can read up on and also lots of post regarding PLEASE HELP WATER MONITOR ie: youll learn alot from what some of the long time monitor caretakers have to say.
Enjoy your water, mines an awsome little critter I wouldnt trade for anything.