The only legal fully aquatic turtle that I know of are Fly River Turtles (aka Pig-Nosed Turtles). Did you see these in a store somewhere? Got a description of what you're looking at?
Fly Rivers have flipper instead of feet. They are the only freshwater turtle with flippers. They reach an adult size of 22" (shell length), so you need a HUGE aquarium and filtration system, and are highly aggressive so you can not put anything else with them, including other turtles of its own kind.
Beautiful turtle and really cool to watch swim, but really not a good choice for a pet.
They seemed to be in every fish store I visits lately. They're so cute and small at 3". I was gonna buy one when lfs told me it will grew just a couple of inches more. Luckily i decided to do my research first and as I suspected they grow really huge and will need a pond eventually