Sea Dragon=Master Otter

Detail of a bas-de-page scene of four men and a woman in a boat that is being attacked by a winged sea monster. Origin: England (London/Westminster or East Anglia?) Attribution: Queen Mary Master Taken off of a Cryptozoology photo-sharing page
http://pinterest.com/kitskyy/cryptozoology/

In this case the Sea-dragon would almost certainly be a Master-Otter or Dobhar-chu and it is shown as being approximately six feet long from snout to vent (full length estimated at 9-10 feet and one of the more conservative estimates) The (underwater and not directly observed) feet would have claws instead of hooves, and the wings would be spurious and added on only "Because dragons are supposed to have wings."  The core representation of the creature-head with snout and ears, beck and body proper) matches most other important accounts and is likely to be the most near to accurately observed and described by the witnesses.

The originating site called this creature a Serra, which ordinarily means something else and is most likely mistakenly applied to this animal in this sighting. A Serra is a creature with a jagged back like a saw, and it is not a name commonly used in England, Germany or France.