Ships of this time period came with two general hull types: Merchant-Built and Frigate-built. The rigging and number of masts a ship had denoted what kind of vessel it was: From smallest to largest the most common ships of the 17th and 18th centuries were: Pinnaces, Sloops, Schooners, Brigs (including Snow and Brigantine rigs), Three-masted Merchantment/East-Indamen, Man-of-War/Ships of the Line. As they were a criminal operation, Caribbean pirates never bought or built their ships (with the exception of landowner turned pirate Stede Bonnet). Any vessel used was either stolen or taken as part of a mutiny. Once taken, the crew would modify the vessel to better suit their needs in the same manner as people today would customize their cars perform the way they want. As hunters at sea, any vessel utilized by the pirate company would need to be as fast and maneuverable as possible. Towards that end, they reduced as much as possible the upper decks commonly found in the fore and aft of the vessel; removing cabins and compartments to not only prevent structures from blocking the wind (and reduce speed as a result), but to suit their egalitarian philosophy of no man being above the rest or having any special privileges (like a cabin all to themselves). Removing the upper structures fore and aft of the ship also allowed for the main deck to be expanded thereby making room for additional guns to be mounted as well as a clearer space to facilitate the movement of ammunition, communications, and snipers (musketeers who would be in charge of picking off enemy crew should it come down to a fight).
Additional modifications could take on the form of changing the sail plan, rigging, even moving or removing a mast. Speed was essential for chasing down prey and escaping pursuit. There was no profit in fighting a naval warship, and every chance of losing to a better-drilled crew that often contained at least a small unit of either Marines or other professional soldiers.
As mentioned previously, pirate crews would look to increase the ships armament as much as possible. While we have seen that the goal was not to fight, it was vital to be able to win any engagement that occurred as captain and crew lived and died by their reputation. Furthermore, a fierce armament went a long way towards convincing a merchant crew to surrender. As such, crews had no problem cutting into the hull to add extra gun ports, swivel guns, and mount cannons of a calibre higher than that ship would normally carry. If the ship was big enough (being at least a 3 masted merchant-type vessel), a crew might add a another whole gundeck beneath the main deck. This would double the ships offensive capabilities allowing it to outclass nearly any merchantmen and even allow it to challenge naval warships of several classes.
Whatever ship a pirate crew utilized the goal was almost always to close the distance with their target vessel and demand their surrender. The goal here was to avoid a fight by simply scaring the prey into submission. Towards that end, many pirate captains and by extension, their crews, would go out of their way to cultivate a fearsome and intimidating reputation. It made good business sense if you could instill such fear into the hearts and minds of merchant captains and their crews as they would be less likely to try to run or worse yet,fight.
While pitched sea battles make for great cinema, the grim reality of naval warfare within the 16th-18th centuries was that even a minor engagement could result in grievous injuries and death amongst both crews. The reason for this was the nature of the ships' construction. When struck by the heavy iron balls that served as ammunition for the ships guns (ie. cannons), the wood would splinter into shrapnel-sized fragments that travelled in all directions maiming and killing everyone in it's path. The medical technology of the time was woefully inadequate to save men from such injuries thus resulting in high levels of disfigurement as limbs were cut off to save the rest of the body from infection--and that was assuming you weren't killed outright by the blast. A pirate captain who was unreasonably violent and aggressive risked not only his ship and crew but the integrity of the target vessels cargo as well. Spoiled cargo, damaged by cannon fire was harder to fence. Members of the pirate crew who took injury in battle were due compensation above their normal share. A damaged pirate vessel, damaged cargo, and injured crew all ate into the profit margin. A pirate crew who took any injuries deemed avoidable suffered from loss of moral and would begin to question the merit in their choice of captain. Therefore, it was in every pirate crews best interest to avoid bloodshed as much as possible.
Crews were a mix of British/Welsh/Irish/Scottish, French, Spanish, Portugese, Native Americans, escaped African slaves. Essentially, anyone willing to make their mark/sign the ships articles were welcomed into the crew.