We have heard the halacha that one can do kriyat shema in the mikva (even if only bshaat hedechak). The principle seems to be the water providing chatzitza. We later find a stira between tzoa and erva covered by glass. We are machmir not to say shema/tefilla with erva covered by glass because the issue of reiya. How would one rationalise the heter on water providing cover? Especially given there is an opinion to stir up the silt, etc. with one's legs to obscure vision of the lower extremities. One must assume the water is clear and not murky and those who do not require the legs stirring up the silt, they must not be choshesh on reiya