Kinosternon (mud turtles) and Sternotherus (musk turtles) do well in a 55 gallon tank. They are strongly aquatic. Some of them will use land areas, and some won't; it just depends on the individual turtle. You could probably keep 2 or 3 in that tank, so long as only one is a male.

The only other turtles that I can think of that would do well in that tank are male map turtles (Graptemys species); they are much smaller than the females and so don't need as much room.