Alison, it would help if you could give us some idea of your location and the current weather patterns. Here on Canada's "Wet Coast" - which is only wet in the winter - it is quite common to have maple trees drop a bunch of leaves in mid to late July as the dry weather catches up. Planting a tree here is best done in early November so it can put lots of roots out in the wet, winter months.
Also, define "watering". Most people don't realize just how long you need so slowly drip water on my "clay" soil for more than the top 1/4 inch to show any moisture. I mostly put the hose on drip and let it run all night. For a new tree I'd probably do that once a week if it's a bad drought and hot weather.
When we first bought our property, the former owner watered *constantly*. I knew that I didn't have the time or energy to do so, and personally feel it's an environmentally bad idea. I gradually, over several years, weaned all the fruit trees off supplemental water with the exception of *really* bad drought years when I often give certain of the higher-risk trees one thorough over-night watering in late August (our bad drought years can carry through to mid-to- late October).
Planting companions around the tree to help keep the soil shaded, building the soil with chop and drop, mulching with woody material every couple of years have all helped to build the soil's water-holding capacity as well.
Thanks for the reply and advice! I live in the mountains of Idaho. It's just recently turned hot and dry. Last few weeks have been 80 during the day, 40-50 at night. Not much rain. Before that, maybe 10 degrees cooler and some rain (decent rain storm 1-3 days per week). I water once a week for 4-5 hours. Just put the hose on a slow steady very gentle flow. It's not normal here for leaves to turn yellow already. It's just started to be summer a few weeks ago! We've got another month of summer and then leaves start to turn. I have cover crops of clover and vetch and such planted around the tree.
Any chance you've given it too much water? Are the roots rotting? Have you considered getting/making some compost tea and watering it with that to help?
Was the tree bare-root or potted? I've read that there's a specific microorganism that Maple trees require. If it was bare root, you might try finding someone with a healthy maple tree and get a bucket of dirt from them?
I agree the tree doesn't look terribly happy in the picture, but I usually don't plant maple trees - Mother Nature plants waaayyyy... more than we can use in some very troublesome spots like our eves troughs! I wish yours luck.