Looking from another perspective: You can see how the pelvis sticks out, and is attached to the spine. I would'nt expect an all water being, to
withstand the amount of stress on the pelvis-spine attachment, without suffering a lot of strain on the joint itselves.
Notice how dolphins (Whales, fish, etc) don't have a pelvis, and can move their spine up and down without doing any strain to joints.
A pelvis would mean a creature couldn't swim properly - Eg. a mermaid.
[edit on 26/9/09 by Thain Esh Kelch]