why would you loose hand speed? its not that you are lifting something or that your muscles are getting heavier.
even some boxers are shadow boxing in pool because of water resistance to improve their hand speed.. (i think its james toney but not sure, too lazy to search
swimming is great to improve cardio and it also strengthens whole body
try pyramid tehnic: 400m any tempo
200m (25m sprint, 25m any tempo and style, its like rest)
100m fastest you can
200m same as 200 above
400m same as 400 above