I am not a biologist but i am quite sertain that the bigger the creature the longer it can hold its breath, this is becouse it has a slower metabolism.
I looked into this a bit, it seems to be true? If so, they'd probably be able to stay underwater for around 9 - 12 hours, which would make oxygen a non-issue and make it so they'd only rarely have to surface. Wonder how that would affect their relationship with land.
