When swimming to something that's on eye level (since you're one block high when swimming) you still hit your head to the ceiling if that's in front of you as if you were the two block high hitbox. This does not occur when you are going up to that ceiling, then it behaves like if you were 1 block high.
Also see: https://i.gyazo.com/baca759c62e0d1933579dbb8a911b39e.gif
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Confirmed in 18w08a.
Passing a block underneath isn't possible in general unless there's at least one block space between your head and the edge.