User blog comment:Blood Reaper/Skyrim Dawnguard Trailer/@comment-2252497-20120531171044/@comment-2252497-20120531204111

Yeah, it may be a set encounter, but I could definitely see Dragons knowing how to dive in deeper water, like landing areas, then coming up to attack you from a different direction or a dragon spawning in water and suddenly diving out. But If that's not the case, it could either be a dragon like Parthurnax but helping you on your quest or Ohdahviing, another companion with unique spawning features near water? Just a stray thought.