This is a story that is hugely reminiscent of the TV Comic tale Shark Bait.  There are odd patches of invisible water which allow sea creatures to seemingly swim in mid-air.  Barbara nearly gets eaten by a giant fish but it is apparently too stupid not to swim outside of it's natural habitat whereupon it dies.  It would be like a fish on Earth just deciding to swim out on to land for no apparent reason.

Decided to do all 16 stories of these I think there basically Ian 1st and Barbara stories the final 2 retell the webplanet with just those 3 but over all there not the best but I think if I was from the 1960s I’d love them