In marine mammals, there are two different body types for two different niches:
A long, strong tail for all-marine whales...
...and all four legs modified into flippers for pinnipeds that feed in the water but breed on land.
So as you can see, many marine mammals have one form or another, but why not both? Couldn't a semi-aquatic mammal have a long, strong tail alongside its modified hindlegs? (Granted, the hindlegs would be smaller, but the point still stands.)