|A naval battle, from a ms. dated late 13th/early 14th century|
At the end of May in 1213, King Philip II of France (mentioned here) was fighting in Flanders (someday I will get to that story). It was known that France thought John weak, and was planning an invasion of England. John decided it was prudent to send his forces to Flanders and try to deal with Philip there, while he was already busy in conflict with someone else.
So John sent 500 ships and 700 knights, along with mercenaries and all the extra servants and other non-combatant personnel that a military campaign requires. His fleet made for the estuary of the river Zwyn on 30 May, where they encountered Philip's fleet, anchored at the town known as Damme. The French fleet was over three times the size of England's; rather than present a problem however, the fleet was manned by a skeleton crew, the military all having gone shore to march to Ghent for their battle.
The English captured a few hundred ships, burned a hundred more; the following day, they did it again, as well as disembarking and attacking the town. Unfortunately, Philip returned to Damme that day, and the English had to flee. They were in possession of hundreds of French ships, however, as well as all the goods that the French nobles carried with them while traveling. One writer of the period claimed "never had so much treasure come into England since the days of King Arthur."*
The damage to the French fleet was considerable, and not just from the deliberate actions of the English: there was so much debris from destroyed ships that the estuary was blocked, and the remaining French fleet could not sail out to open water. Philip had to abandon or burn the remainder of his ships.
*The biographer of William Marshall, in L'Histoire de Guillaume le Maréchal ["The History of William the Marshall"]