I agree with nitrowixen, swiming can be your cardio. And yes, you do need a waterproof watch. (Better: a waterproof heart rate monitor?) If you have the proper technique/speed you can do the same hiit. According my experience, there is one difference. When I swim I need a longer "warm up" time, than with any other cardio, before starting the hiit.
And I agree with Wee, try to find a pool where there are different lanes for different level swimming, so slower swimmers do not hold your back.