Martin is 100% correct, and the reasoning is simple - water contains less oxygen than air. Species which respire mostly via water either have gills (which have sophisticated counter-current blood flow to maximize extraction of O2), or vastly expanded skin surfaces (as in the Lake Titicaca frog and hellbender salamanders) and live in cold, fast-flowing waters (cold water holds more O2, fast flow improves gas exchange). That's why you get fish die-offs in hot, stagnant ponds - hot water has less oxygen, still water doesn't exchange it as well with the atmosphere or mix between water layers, and it eventually reaches a point that even the highly specialized gills of fish cannot extract enough O2 to stay alive.

As far as swimming ability, I've actually never run across a healthy frog that cannot swim, and for the past 6 months I've explicitly been tossing every species I can get my hands on into a large tub of water and filming their swimming with a high-speed video camera as part of my thesis. I've not only gotten Hylid tree frogs to swim extremely well, I've gotten good swimming from spadefoot toads, fowler's toads, and even red & black walking frogs (Phrynomantis bifasicatus). The only one that doesn't swim well is Bufo asper, and that's just because they prefer to play dead, and will swim perfectly well once I turn my back. I haven't done the tests yet, but the bumblebee toads seem perfectly capable of swimming too.