Have you consistently been losing ten pounds a week or is it just your first week? Looking at your ticker, it seems like you've just started -- if so, it's entirely natural to lose a ton of water in the beginning. Not to worry, it's not muscle loss.
However, you're wise to be concerned about muscle loss. Studies show that up to 40% of the weight we lose on a traditional diet is muscle, not fat, which lowers our metabolisms (and makes us look floppy and flabby!
The way to avoid losing muscle along with fat is not to eat more calories - more calories aren't going to protect your muscle, just slow down your weight loss. The way to sustain and protect your muscle mass is to incorporate muscle building exercise into your exercise program. I see that you're doing lots of cardio but you didn't say anything about weightlifting/resistance training. I'd strongly advise you to add that into your progam if you aren't already. You say that you've done it in the past, so it's shouldn't be hard for you to incorporate that into your plan.
Even though my calories were low during the year I was losing weight (started at 1600, ended up at 1200), I was able to maintain my muscle mass and even add on six pounds of muscle. I lifted weights five days a week and know that's the reason why.
If I'm reading you right, you're doing 2 - 2.5 hours of cardio (swimming plus other cardio) plus
running 5 - 12 miles a day? I think that's excessive and may be counterproductive. You're going to overtrain at that rate. How about switching some of that cardio time to weights?
Best of luck to you -- it sounds like your off to a great start!