The only time you will be able to see any kind of beach in the city itself is when the Mississippi River gets really low then you can see a beach.
There is a beach out at the lakefront of Lake Pontchartrain. This is an undeveloped area and used to be part of the amusement park Pontchartrain Beach which was demolished in the 1980’s.
There are some nice beaches about an hour away on the Mississippi Gulf Coast where you’ll find communities like Pass Christian, Gulfport and Biloxi which is now famous for excellent casinos. The water though is not very swimmable because it is very shallow and also it is water from the Mississippi sound which is not clear at all.
One could take a tour boat out to Ship Island that leaves from Gulfport for a day trip where you will find a really nice beach and clear water to swim.