You should be able to put in your time, but no matter how you enter it, it might not be very accurate. The problem is that all of the walking entries are for walking on land, which is much easier than walking in water. So even though you're going slower and you're working hard, you aren't going to get as much credit for it b/c those are land entries. I'm not sure of a good way to estimate those calories burned unless you wear a heart rate monitor during your workout.
Sorry I'm not more help,