First, one of the big difficulties with using a fluid medium to "breath" is that the muscular effort to move it is exhausting...The mice that they left in there for too long (more than a few minutes) died from that alone.

Second, the big problem with using Bypass or ECMO is that it requires anticoagulation...So, we're going to inject you with heparin, so you can't make a clot on a dare, and then shoot you into space! I don't see that ending well!

I should add that those are both problems that can be potentially solved. But they will require fairly big advances to pull them off.