As someone who does live in Maryland and will be paying this tax, I'm of two minds about it.
On the one hand, I think that the basic idea is good. Reducing soil run-off and protecting the Chesapeake is critical. That costs money, and it makes some sense to collect that money proportionally from those who are most contributing to the problem. So the big box store with the giant parking lot has to chip in more than the guy in the tiny townhouse. Sounds good. And you've got a chance here to educate people about the issue and teach them about things like dry wells. Bonus.
On the other hand, this is a pretty good example of a government program that goes into place and then leads to a snowball bureaucracy. It's all good intentions, all the way down. First someone has to examine satellite photos of the entire state, measuring rooftop and parking lot / driveway areas. I'm sure much of that can be done algorithmically, but I'd bet a dollar there's at least one guy out there in a cubicle working 9-5 drawing little rectangles on maps just for this. Anyway, that's a one-time thing, except for new construction.
Then there's the environmentally conscious company that collects its runoff and puts it to good use. They should get rewarded for that, so they get a credit, which means there's a process for documenting what they're doing, and verifying it. Great -- we get more companies putting in rain gardens and permeable pavement. You want to encourage that, and traditionally changes to the tax code work better as a motivator than either inspirational marketing campaigns or punitive fines. So we're good there.
Then the guy who spent a lot of time putting in a rooftop garden on his house because he's very concerned about the run-off issue says "Hey, how come there are tax breaks for big companies but not for the little guy?". You're right, little guy, that's not fair! So the law gets modified; there's a residential credit application process where any homeowner can document what they're doing, and get credit for it. There's a spreadsheet to download, and you just have to submit a bunch of photographs along with that spreadsheet to prove that you're doing it right. (Really: http://www6.montgomerycountymd.gov/Content/dep/downloads/water/Residential_Credit_Application_and_Credit_Calculator.xls
) There's a page in there to detail your Maintenance Checklist, where you state how many times a year you inspect your rain barrels to make sure they don't have more than two inches of standing water.
So that's fine. Of course there's an appeals process in case you submit your application and it gets turned down, because otherwise it's unfair, and you don't want to send every appeal over into the overworked court system. Also, you need to reapply every three years, in case things have changed. And for each step in this process there's someone working in the bureaucracy to develop these forms, to process them, to deal with the appeals, to answer questions on the helpline, etc.
And all of this varies county by county, because out of fairness you can't expect the poorer counties to pay as much as the wealthier counties. Plus that compromise had to be put in to pass the legislation in the first place.
Bottom line is as a typical homeowner I'm paying an extra $88.40 bucks a year (There's a website to look it up. It even has pictures. I can see my house from here!), and I guarantee I've spent more time looking into this than most people. Most people will get an extra charge on their water bill and just pay it, maybe grumbling in general about taxes. Others will look into how they can get that credit, maybe buy a couple of rain barrels or something, and eventually conclude it's not really worth it. (Buying two $50 rain barrels and hooking them up to my drain spouts will save me... drumroll.... 60 cents a year off of my tax bill, as it turns out.) It's probably still a good idea to get them, but the tax system here isn't really providing an incentive to do it.
I can only imagine what percentage of the money collected under this tax will actually go to projects that help protect the bay. My guess is there's a pretty high overhead rate here, which tends to be the case for these small issue-focused taxes. But they're a lot easier to get through the legislature than a more general income tax increase. And once they're in place they will almost never go away. It seems to me like you could probably collect more money with far less waste by just slapping a $20 "Save the Bay" service fee on everyone's water bill regardless of square footage or protective measures taken, but admittedly that's just a wild guess.
Anyway, that's how sausage gets made. Mmmm. Yummy, yummy sausage.