SS,
I think you'll find that Harford Co. has more of what you're looking for, but obviously you'll drive further.
Dog Park Info:
Maryland Dog Parks and Dog Exercise Areas ~ MarylandPet.com (You'll find one in Bel Air in Harf. Co., but none in Cecil Co.)
There's an indoor/outdoor tennis center in Bel Air, but none in Cecil Co. that I know of. Might want to check with the Cecil Co. Community College to see what they have going. Often times Comm. Colleges will offer non-credit tennis classes where you might meet peope.
Cecil Community College
The YMCA in Elkton is supposed to be nice.
YMCA of Cecil County, Welcome to our website!
Chantilly Manor CC in Rising Run in Cecil Co. used to have tennis courts and a pool, but they were bought recently, and I don't see them mentioned on the new website.
Chesapeake Bay Golf Club Welcome Page Might want to call to see if they still have them. Newark CC in Newark, DE has a pool, but I'm not sure about tennis courts. Maryland Golf & CC in Bel Air and Swan Creek Golf & CC in Havre de Grace have both, with the former being much nice and much more expensive.
For boating interests, Havre de Grace is a hot-spot in Harf. Co., but I'm not a boater so I can't help out there.
Here's the Cecil Co. Community Board where you might find more info:
Cecil County Magazine - Cecil County MD Tourism Information (broken link)
You're doing the right thing in trying to meet people through common interests, but I just don't know how organized things are up there for your interests. I moved to Harford County 17 years ago and didn't know a soul. I made good and lasting friendships by joining a golf course, a scuba club, meeting people through work and through volunteer activities. I also just recently took up kayaking and have met some really nice people through the local nature center here that gives classes and promotes the sport.
Good luck.