What dinosaurs were known to be bad swimmers?

Answer 1

It's doubtful any dinosaurs could swim very well. The reason is that there legs are positioned to carry their weight efficiently against gravity, by being directly beneath their bodies. Swimming creatures which have four limbs tend to have them more off to the sides. It's possible, though, that some dinos whose diets consisted of mostly fish (like Baronyx) were able to negotiate short stretches of water.

The great swimming reptiles, like pliosaurs and icthyosaurs, were NOT dinosaurs.

Actually, there is evidence that at least two species of Hadrosaurs were fairly good swimmers - their fossils show carbon impressions indicating they had webbed feet.