I can sometimes float in sweet water, but it is not as relaxing.As you know, it's very difficult to float in fresh water in a way that you can breath and stay absolutely still and your mouth and nose comfortably out of the water. The fish asana is one of them (actually it's the only way I know). (Floating is salt-water is much easier.)
Floating in 'the fish' takes some getting used-to and is quite counter-intuitive. One thing is that you are arching your head
back, almost as if you are pushing it into the water, and indeed your ears and eyes(?) and top of your face goes into the water. Only a certain amount of your body can stay out of the water, because of the law of flotation, and by letting most of the back of your head and upper face into the water, this is what
allows your nose and mouth to stay well out of the water. You keep an arched back and your upper lungs full of air, breathing into your lower lungs.
But when you get used to it, its really lovely....
It doesn't much matter what you do with your legs, as long as you keep them under water. Keeping them flat is fine, and you can also let them go limp, but they can also be put in the full-lotus if you can do that (there will be enough friction in your thighs to keep them there).