first, I would like to ask whether you wear contact lens or goggle as you swim. Both of them can induce diplopia (double vision). Try to swim without them and see if the diplopia returns. If it is not, that is ok, just transient double vision. But if you still get diplopia, you'd better get to an Ophthalmologist or Neurologist ASAP. There are many eye diseases that are result from bacteria and viruses in pool water. Have your double vision checked out soon.
Of course, it is not normal to have double vision after the swimming. During the swimming, you'd better wear the swimming goggles to protect your eyes from the bacterium infection. If you don't wear it, you'd better wash your eyes after the swimming. However, the double vision may be caused by the immense increase eye pressures. Thus, you'd better go to see the doctor and have the check.