The oxygen isotope ratios for _Spinosaurus_ indicate that the body was immersed much of the time. Immersion in water reduces aerial evaporation and therefore decreases enrichment of 18-oxygen in body water.
If it stood in water a lot, it was able to drink any time it wanted; and its food came from freshwater, too. Finally, how much water evaporates through sauropsid skin, and how much enters it by osmosis when a sauropsid is immersed?
In total, I don't see a reason to expect different oxygen isotope ratios in spinosaurs and aquatic or semiaquatic crocodiles, unless metabolism has an influence (which, apparently, it doesn't).
I'll reply to the biomechanics stuff later, I must urgently finish a grant proposal...