Yes, you can tint Envirotex. I use a drop or two of cheap acrylic craft paint per ounce of mixed Envirotex. That leaves the "water" colored a bit, but still pretty clear. It will harden just fine. I've also mixed in considerably more paint, to the point where it was opaque. At this point, though, it looked more like green glass than water.
You should use multiple thin pours. I like about an eighth of an inch per pour. If you pour it too deep, Envirotex will cure with bubbles embedded in the "water." Unless your are modelling a hot spring or waterfall, this will not look right. I tint each of my pours differently - darker blue at the bottom up to light green or clear at the top. The water looks more realistic this way, as it gets slightly "cloudy" towards the bottom.
I've never used WS Water. Some people say that it doesn't ever completely harden, and it attracts dust more than Envirotex. Since I've been happy with Envirotex, I see no reason to switch. You can use WS Water Effects (a different product) on top of Envirotex.
You can get Envirotex at craft stores like Michaels or A.C. Moore. Go to www.acmoore.com and find the coupon. You'll usually find one for 40 or 50 percent off one single item. Since Envirotex is relatively pricey, this will save you a lot of money. Michaels will accept the A.C. Moore coupon.