@daisymae
I should have mentioned this to you before. Maybe I already did and forgot.
It’s more difficult to know how much you are losing from sweat when you are swimming compared to running. I finish most of my runs dripping wet. I bet you finish all of your swims that way. 
But what you should do is follow the sweat-loss calculations I have mentioned on a different thread. Weigh yourself before swimming, and then immediately after you have finished. Add the weight of the water you drank while swimming and you can calculate how much you are losing in your workout.
Here are detailed steps on calculating it:
figure-out-your-sweat-loss
This can give you an idea of how much you should be drinking to replace it!