I think it's 2 hours, 10 minutes, and 26 seconds.

We know that:
d = rt

So,
5 = 2.3t
t = 5/2.3
t = 2.173913043

So, 2 hours and a decimal remainder we need to convert to minutes and seconds (which are base 60!)

0.173913 hours * 60 minutes in an hour = 10.43478

So, add 10 minutes to those two hours.

Again we have to deal with a decimal remainder. Let's convert that to seconds:

0.43478 minutes * 60 seconds in a minute = 26.0868, so about 26 seconds.

Put it all together:
2 hours, 10 minutes, 26 seconds and a remainder.

Now, obviously, you'd have to be some kind of robot shark to swim this kind of consistency. Real world waves, your own fatigue, etc. would mean that the difference between my answer and Michael D's would probably be within the bounds of your actual swimming abilities.