Q: is iwatch have sim card in it? how do i see call while im at the water?

The Apple Watch does not contact a SIM. It is not a cellular device. You use your paired iPhone to make and receive calls. I don't suspect you will be making calls in the water, since you need to be in range of the iPhone.

Neither generation of Apple Watch has a sim slot.

You will see notifications while you're in the water only if your Apple Watch is able to connect to your iPhone.