It's a lot of energy to make those oceans, on the order of decades of the total solar radiation that Mars receives (my SWAG is over 10 years just to get ice pas

It's a lot of energy to make those oceans, on the order of decades of the total solar radiation that Mars receives (my SWAG is over 10 years just to get ice past the melting point). Something like 5E16 KJ, plus whatever you need to warm the ice up to 0C. Of course, this isn't practical. Once you do that, then, yeah. Water vapor rises, gets slammed by UV radiation and dissociated, and hydrogen goes thataway (not sure what happens to the oxygen, maybe it gets bound up in the soil?). It's pretty much how Mars lost its atmosphere to begin with. Not having a magnetic field is bad for the environment.