Death's Daughter is the name of the Dead Pool's (Gangplank's now-destroyed ship he took from his father) old main cannon.
It is speculated Gangplank now sails on the Leviathan (the Noxian flagship he stole from Swain) considering Miss Fortune orchestrated the Dead Pool's destruction with him on board and yet he still calls down a Cannon Barrage in-game.
Gangplank and Illaoi were on a relationship over a decade ago, but they went their separate ways.
After subjecting him to a Test of Spirit in order to get him back 'in motion', the Kraken Priestess ordered the Serpent Islander chiefs to support the Saltwater Scourge in retaking Bilgewater.
Illaoi was Gangplank's first love.
Swain and Zed want Gangplank dead for stealing from each of them.
Miss Fortune managed to overthrow and 'kill' Gangplank in revenge for him murdering her mother back when she was a girl.
Gangplank just thought of Miss Fortune as the most persistent of bounty hunters (now the architect of his fall) always unaware she is the daughter of the gunsmith he killed when she refused to hand over the twin pistols he had commissioned in order to spearhead his rise to Reaver King of Bilgewater.
At one point the Saltwater Scourge and the Bounty Hunter struck an uneasy truce in order to fight off the Harrowing, right on the shores of the Serpent Isles, in 'The Battle of Knife Straits'.
They were successful (despite heavy casualties and their combined fleet scattering) while farther south Nautilus managed to fight off the rest of the undead.