No Swimming Animation, Either for Humans or Mermaids!

I bought Sims 3: Island Paradise a few days ago hoping to explore the underwater lots, but when I finally made a mermaid in my game, she would enter the water, switch to having a tail, and then WALK atop the water as if she was taking a leisurely stroll. Initially I thought this was a glitch specific to mermaids, until I finally got to an underwater lot and saw a scuba-diving normal sim, who also was walking around underwater instead of swimming. After further inspection, it appears that any time a sim is in water, regardless of whether they're swimming on the surface or underwater, and regardless of whether they are human or a mermaid, the swimming animation does not work at all. My sims can only walk. If anyone knows a fix for this PLEASE let me know. Otherwise, I'll try to get a refund, because there's really no point having an underwater/mermaid pack if the sims can't swim properly.