Karen Gillan Needs to Learn to Swim!
Apparently the only reason we didn’t get to see the TARDIS swimming pool room in The Doctor’s Wife is because Karen Gillan doesn’t know how to swim!
“Making television - and especially making Doctor Who, where you have to imagine a new world, in my case a new universe, with every episode - is always a negotiation between dream and reality. (That was the same email in which I learned that we weren’t going to have the scene in the Tardis Swimming Pool because Karen Gillan couldn’t swim.)”
Source: Neil Gaiman’s Journal: A Fairly Humongous Doctor Who Q&A Mostly
(Scroll down to the question about “Why an Ood”)