The only sailboats that can safely navigate the Susquehanna Flats are Hobie-Cats! I've been sailing this part of the bay for 7 years and never ran aground. However, I see a couple boats hard aground every year. I would venture a guess that nearly all of they were either not paying attention to where they were going, or they didn't have a GPS/Plotter onboard.
Getting to Havre de Grace is easy as long as you pay strict attention to the channel. Keep in mind that massive, deep-draft barges traverse this channel every day, barges loaded down with huge boulders, mountains of gravel, and stone, materials used to construct jetties and stone bulkheads. Some of the barges draw 12 to 15 feet of water and the tugs probably draw at least 6-feet.
South of the Susquehanna Flats you'll find some great sailing areas, lots of room, not many crab pots, and some neat, sheltered anchorages to spend the night. If the wind's blowing from the south or east, Still Pond is a great anchorage. The Havre de Grace basin, the ditch in front of the VA Hospital at Perryville, the Spoils Islands south of Havre de Grace, several locations in North East River's upper reaches, the Elk River, much of the Sassafras River, Bohemia River and many, many other lesser known locations are all accessible with a 5-5 draft sailboat.
One of the best sailboat races I've seen is the Thursday evening races in front of Havre de Grace. Some really fast boats there and the races are always close. I'm not a sailboat racing fan, but this particular race is really neat - mostly Lightnings, Stars, and similar makes.
Hope this helps,