Breeches made from fine unbleached linen closed with a fingerloop braided cord. With additional slits at the waistband to attach hose. Breeches (or Braies) are an ubiquitous piece of underwear in the medieval period.
Even if you are a lady pirate you better wear pants at sea. For example these knee-length blue and white striped pants. If you are not very comfortable at sea, you can also use these pants for a medieval costume.