I haven't taken the CPESC or CPSWQ, but I know someone who has, and I have researched it quite a bit. I think you will learn a LOT of specifics studying for those certifications.
But VT is right - the PE license lets you do all that stuff anyway, so there's little incentive to go after those additional certifications. There's a reason those certifications exist, though - as you well know - the PE exam doesn't cover erosion control or stormwater BMPs at all. Because of that, a lot of State regualtory agencies (including my own, small as it is) are beginning to require additional certifications for designers of stormwater systems and erosion control plans.
As a plan reviewer, I fully support requiring those additional certifications. The plans I have seen submitted by the guy I know with the CPESC designation are a world apart from the plans submitted by his competitors - they sail right through the permitting process. Meanwhile, I continue to get crap from licensed PEs that simply will not work or are full of errors, and I have to turn them back. Usually this is met with "Hey! I'm a PE! I've been designing parking lots since the 1970s! You can't tell me that I'm wrong! I have a license to do this!"
Yep, I like those certifications, and I am going to get them as soon as I am able, and I am looking seriously at requiring them for site designers around here (or at the very minimum, some sort of home-grown certification).