About a mile long, in the Assunpink WMA. To the east you can find two other small lakes in the same watershed: Stone Tavern Lake, and Rising Sun Lake. Assunpink Creek drains into Lake Mercer, which is much larger than any of these.
Shark River is not really a river, it is more of a saltwater lagoon with an outlet to the sea. The actual inlet would be a very bad place to go, with a lot of boat traffic and sometimes swift tidal current. The lagoon is much better, and I have marked two spots that are easy access and parking - both in public parks. Don't try to use the boat ramp, you might get in trouble.
Don't spend a fortune on a PFD, or Personal Flotation Device, commonly known as a life preserver. Unless you plan to go far offshore in deep water, you will never need it or wear it. Most of the places I go, self-rescue would be a matter of standing up, or a couple of strokes to shallow water or the shore.