Fish receive oxygen needed from water that passes through their gills. If fish were isotonic to their watery environment, what problems would this pose to them?
Some fish are isotonic to their environment (I read that in some book and they had examples but Google is failing me. Here is a paper instead talking about fish doing well in an isotonic environment). It isn't an issue for getting oxygen; I assume you think that because you think fish get oxygen via osmotic pressure, but "isotonic" (and "hypotonic" and "hypertonic") refer to the transfer of water via osmosis. A fish that's isotonic with its environment doesn't need to take special measures to avoid its cells swelling or shrinking as water floods in or out of them; it doesn't say anything about which specific solutes are in that water to make it isotonic.
As far as oxygen goes, oxygen is constantly being consumed by the fish's tissues so the diffusion is always going to go in the right direction (unless the water doesn't contain enough oxygen in the first place, in which case fish won't survive). In addition I imagine fish blood contains many molecules that preferentially bind to oxygen in ways that make this process even more efficient.