Ships and Naval Warfare
Moving by sea was a dangerous, expensive business. Ships in the period were sturdy and seaworthy, but had poor sailing characteristics compared to those of the late 15th century, and navigational techniques were not yet on a scientific basis, although the compass and astroble were in use. By 1492 Christopher Columbus was, in fact, sailing with the aid of the very latest and quite recently developed rigging and navigating technology, but was still a better practical navigator than a scientific one (all of his star shoots were off by a consistent error).
Medieval merchant ships (with crews) were expensive to hire and not very dependable in getting your from here to there. Ships in the period "averaged" 60-100 "tuns," which was a measure of capacity (the number of barrels of a certain size which could be carried, so in practice we have to think of these as being a bit more than a modern measurement ton). Assuming a "typical" vessel to be about 80 tons, 90 rental would have been something like 155,000 ducats. A fully equipped and manned war galley, such as Venice, Leon, or some of the other Mediterranean states owned, could rent for about 385,000 ducats a quarter. The merchantman would have had a crew of about 30 men, the war galley at least 100.
Capacity seems to have depended upon circumstance. For transport purposes it seems that most ships could carry only about one passenger per ton (for our purposes c. 60-80 men, as our "average" ship is 60-80 tons). But it was not uncommon to cram lots more men aboard for battles. There has been occassional mention of 400 men.
A crossing of the Channel would take a day, or even weeks, depending on the wind and the weather. Moving a large number of troops at once was even more difficult. This, however, was usually neccessary because you didn't want your troops to land on the enemy shore in small groups, there to be stomped on by larger bodies of hostile soldiers.
Fleets could be concentrated fairly quickly. On 10 June 1340 Edward III issued a call for ships. He had in hand on that date 40 vessels at Harwich. At the end of 10 days he had an additional 10 at Harwich, plus 200 at London.
Except at Sluys (23 June 1340) and Damme (June 1213), fleets were small, 100 vessels being rare, and 50 or so the norm. At the two named battles the English had 250 and 200 ships respectively, while their foes had 500 and 400.
Also note that the earliest recorded mention of cannon on ships was in 1336, an English ship operating in the Channel carrying four rather primitive pieces. It would be two centuries before ship building and cannon technology matured sufficiently to make the combination cost effective.