Calypso, if you're considering the northern Chesapeake then look into Havre D' Grace or the marinas at Georgetown on the Sassafras River. We live in Hatfield and it used to take us 1:45 to get to Skipjack Cove. There are a half dozen other marinas within spitting distance, many with pools for the kids. The Sassafras is a beautiful river but it will take you apx 45 mins to get to more open water at the rivers mouth. Another 30 mins to actually reach the bay. While you can sail on the river it's narrow and the power boat traffic is constant. On the plus side are the beaches across from Skipjack where you can dinghy the kids, dog, family to shallow water and enjoy a swim, cold beer, etc. Not to mention some of the best starry night skies and gorgeous sunsets. I mentioned Havre D' Grace and it too is a nice place but more limited in beach areas to go play on. It too is a long way to more open water. Not sure what impact the recent storms and all the silt flow will have on the Susquehanna flats but I'm betting it wont be an improvement that's for sure. That I know of there's really only one marina, Tidewater Marina, unless you go upstream to Port Deposit. Others on the site will be able to give more detailed info on options on the area. Good luck with your search.
Mike