Water Walking

A lot! There are too many factors to just throw out a guess (your weight, how deep, waves, walking speed). I can tell you it's one of my all time favorite strength exercises in the summer (resistance!). I'll walk in ocean water just below the knee. 20 minutes will slay me, and I've got some really strong legs. I'll go deeper just for a little break. I recall reading the deeper water made it more difficult, but that's not my experience at all. Deeper water makes you feel like you're floating on the surface of the moon. I'd say for the latter add 20% to what you normally burn on land. The calorie burn my way is anybody's guess, but your thighs will thank you.

The most accurate and realistic estimate is around 11 calories per minute.
I do it often and when I started I was 21% body fat @ 195pounds.
I class one hour as 710cals but I also do one vigorous freestyle length for every 2 walking lengths.

Of course it does vary and if you could provide us with additional info on your weight, body fat, height, pool depth etc that would help but you won't go far wrong with the above guide.

-Blake