It has long been known that on occasion creatures will somehow slip passed a fortification in a water (or magma) channel. Some people believe these creatures are
pushed through by the current, others believe they can freely
swim through. Well at long last, thanks to the testing arena, I think I can provide the answer:
Both, in different circumstances.
Here's what I discovered, and how. I assumed control of an aligator in a
custom made arena, with water flowing in a horseshoe shape through some fortifications. I quickly discovered that, yes indeed, water can and did push my through fortifications, however I was not able to move through voluntarily. So I thought that settled it, and knowing that only diffusing water I set about creating a creature proof-barrier. The plan was to put it at the bottom of a U-pipe so the water would always be 7/7. But what's this? Now I can swim right through! Apparently if (and only if) the water is 7/7, creatures can simply swim over the fortification.
Also if we assume that fortifications represent castle-wall–style crenelations, swimming over them in deep water isn't necessarily a bug, but the pushing effect shallow running water pretty clearly is.
Unfortunately it is not possible to avoid both of those situations at once. Nor does installing multiple layers of fortifications provide any added protection. Now an interesting question is how these limitations apply to vertical bars, horizontal bars and grates. But those can't be placed the arena, so someone will have to test it in Dwarf Mode, or perhaps Adventure Mode after building special testing area previously.