Hand numb at end of the swim

question...

if you put your arms over your head (like you are raising your hand in class).  do they feel like they go to sleep in a very short time?  was the hand that went to sleep in your HIM your writing hand?

If the answer(s) is yes, could possibly be Thoracic Outlet Syndrome.  I'm no doctor so please take what i say with a grain of salt.  I only know if the condition because I have it and mine was a rare/extreme side of the diagnosis.

If it continues to be bothersome I would go and see a doctor about it.

I would second the thoracic outlet syndrome possibility. If you do a lot of upper body overhead stuff then it can cause it. Theres a few different types where either tight muscles or bones impinges on the neurovascular bundle causing numbness tingling, and sometimes decreased blood flow and other problems. 

Look up Roos test, Adson test, and the Allen test online give them a shot and if positive go see a doctor or just go see a doctor regardless because they would probaly be more skilled at diagnosing and performing the tests than you would be on yourself.