Is that a kit from DeFalco's? I assume you live in Houston? If so, then howdy neighbor! :rockin:
Anyway, I have a couple of thoughts for you.
You first said "instructions say around 1.040". This recipe lists at 1.047. So, it's a 17 point drop (not 10), which is quite significant, and will definitely give your beer a thinner body and lighter flavor. ABV will be around 2.3-2.5%, which will certainly impact flavor as well. To be blunt, it won't be dump worthy, but it won't have the fullest flavor either.
If you have the equipment for it then a blend would be one option for you. To make it simple, make a new batch (correct water this time), and blend with the first beer after fermentation is complete for both beers. You'll of course need a large enough vessel to blend both volumes prior to bottling. Unfortunately, you still won't end up with an ABV that matches the original recipes 4.6% target. The blend will be closer to 3.25% or so if you started with a 2.4% of 3gal and a 4.6% of 2gal. It'll still be thin.
There are certainly other ways of blending, but are a bit more complicated when it comes to math, and may be beyond your equipment or experience level for right now. Of course, I could be wrong in that assumption, and if so, then by all means, take a look at the information available out there. Be prepared to do some math and build your own recipe that accomplishes your goal instead of buying a kit, though.
Of course, you can proceed as is, and blend your thin wheat beer with some lemonade for a tasty Shandy.
