Good points, but continuing to play devil's advocate here, almost all animals know instinctively how to swim, even those who loathe contact with water. I'm sure predatory dinos could swim, just like lions do, but I doubt they pursued prey into the water. A dinosaur might be unbalanced for swimming, but it could still restrict itself to relatively shallow water and walk on the bottom, offering itself support and some measure of protection. This is how I believe these big saurians actually lived, like hippos do today.

I do disagree with you about the thermal regulation potential of water. Alligators and crocs today evade cold snaps in tropical climates by going into the water, which may be warmer than the surrounding air. They also can go into water to cool off on hot days. Water has a high thermal inertia (it's temperature lags the land's) , and blocks sunlight. A pond or lake with islands and banks available for sunning is ideal habitat for big tropical reptiles because of the close proximity of two totally different temperature extremes.