Flagbag,
There is a public boat launch on the west side of Sandusky, downtown. Look for the big coal dock; it's just east of that. Or drive along the waterfront downtown until you get to the west side of town. There is also a public boat launch on the north side of the bay at a little park, but you'd have to drive around the west side of the bay to get there. There are some tricky sandbars and shoals in the bay and entrance, especially across from Cedar Point, so please consult a chart. There is a West Marine downtown a block back from the water.
A good place to swim is on the west side of the big sandbar across from Cedar Point. Drop anchor close to shore and swim ashore. The bottom can be soft so make sure your anchor is set.
You might also want to consider Lake Pymatuning in NE Ohio along the PA border. It's a big lake, plenty big enough for a 17' boat for a weekend. Some of the reservoirs around Canton and Columbus also have sailboats up to 22', such as Atwood Lake.