My theory on these swimming characters: (Short version, they will be VERY restricted)
Dory and characters marked as Swimming characters will ONLY be usable in their Play Set and specifically marked "Underwater" toy boxes. Underwater play sets will function similarly to regular sets, except there's no gravity and an 'underwater' filter will be put over the camera. Swimming characters won't be able to interact with vehicles, ledges, etc. (Maybe they'll be able to use weapons by having them magically floating in front of them or something)
Regular walking/driving figures will not be able to be used in Underwater toy boxes. When you put one on the base a message will pop up with "This figure cannot be used in this toy box" just like now when you set a box to have no fliers, Spider-man only, etc.
No previously released characters will be able to swim, and it'll be counted as a special ability going forward like swinging and wall climbing. Moana or Ariel will be announced as the first characters that can be used both on land AND underwater.
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Obviously, I have basically nothing to base this on - this is just me theorizing!