Keep Sneak and Invisible up due to the Chigoes and Treants in the Mire. Use caution around the imps as they have True Sight. If you want to avoid a few mobs, it is recommended doing this during the day as the Jnun in Caedarva Mire are asleep during this time (although can also be avoided easily during the day).
You will need a Lamian Fang Key (or a Thief using Thief's Tools) to open an Irongate inside Arrapago Reef. You may farm this key either in Caedarva Mire or Arrapago Reef or you can obtain it once per Vana'diel day per character by simply checking a ??? in the Mire (H/I-7) beneath a ledge at the back of a small bog; you can reach the key from the ledge without having to trudge through the muck.
Once inside the Reef, travel north in the tunnel until you see a ship. Board and walk through it, crossing the plank, and travel down the eastern shore.
Remain alert of any Lamiae (detect by sight) and Draugars (detect by sound) that may agro you in the reef.
Travel Northeast to a cave and the Iron Gate (J-10, Lamian Fang Key required). If you are traveling in a group, just one key will be enough to let everyone pass.
Once you go through the first gate, turn left and you will find another Iron Gate (H-9) guarded by a ghost mob (Sneak). A Lamia Fang Key is not required to open this gate, just drop Invisible and open it. (Make sure not to have low HP, or the ghost will aggro you.)
Once you're past the gate, the Corsair ??? is ahead on a ship (H-10).
There is a Lamia on the ship, be cautious when dropping invisible to start the cutscene.
The quickest way is via Home Point if you have already been to Nashmau.
You do not have to go back through Whitegate to return to Nashmau. You can backtrack along the path through the Iron Gates and exit into Caedarva Mire at I-12 of Arrapago Reef Map 1 (where you just did your cutscene). This will take you to I-6 of Caedarva Mire Map 1, where you can head to Nashmau by going mostly south or hugging the right wall. Please take note of aggroing Treants and wandering true sight Imps, before you reach Nashmau.
You must pass through a cemetery (E-10) to reach the zone, there are Imps (True Sight) in the cemetery. During night hours, the Qiqirn turn into undead, so if you can avoid the imps, sneak is still advised to avoid aggroing undead. Watch HP (blood) aggro, also.
Note: There are actually two caves through the graveyard, take the northernmost one (with a Home Point inside).
Once inside the cove, travel along the left shore until you find a ??? (located on a big rock outside a tunnel).