D’Artagnan was worn out with fatigue. A mattress was laid upon the deck for him. He threw himself upon it, and fell asleep.
On the morrow, at break of day, they were still three or four leagues from the coast of England. The breeze had been so light all night, they had made but little progress. At ten o’clock the vessel cast anchor in the harbor of Dover, and at half past ten d’Artagnan placed his foot on English land, crying, ‘Here I am at last!’
Alexandre Dumas, The Three Musketeers, 1844
On his way to London to retrieve the jewels of Queen Anne, D’Artagnan spends the night on the deck of a boat. When he wakes up three or four leagues from the coast of England, he would have been able to see the white cliffs of Kent, and perhaps the distant shape of the Dover Castle behind the morning sea fog (4 nautical leagues equal 12 nautical miles, the shortest distance across the strait is about 18 nautical miles).
The Channel had been known for the presence of its pirates, potentially ready to attack and ransom any boats of their liking. France and England did not have a navy at the time, causing inability to control the seas and protect their coasts. Richelieu created the French Royal Navy in October 1626, while the British Royal Navy was established in 1660, following the restoration of King Charles II. Defeating pirates (and corsairs, privateers, etc…) still took a long time and was very expensive.
Between 1625 and 1637, for example, Flemish admiral Jacob Collaart was responsible for the capture or destruction of at least 150 fishing vessels, bringing 945 captured sailors back to his base for ransom, in Dunkirk. In spite of his rank of admiral and allegiance to the Habsburg, many agreed that he followed a piratical career. Collaart was captured in 1636 by Johan Evertsen (another admiral under the Dutch), and died one year later in Spain. For more pirates from the English Channel, try this.