Why don't they have two black boxes?
The added weight, space, and complexity of a second recorder have to be taken into account, and all are very important for an airplane.

If you are just interested in location, then there could just be a simple beacon designed to float. Although less, this will still have the above costs. And you have to design something light enough to float, sturdy enough to survive an extremely violent crash, yet still be a functional electronic device.

Considering that the situation of a lost aircraft at sea is fairly rare, it is questionable whether the benefits justify the costs.