This daily mission needs to be unlocked once by completing two of the Dread Saints missions in the respective port. The first pre-mission 'When the Saints Go Marching In' is a level 40 ship combat and swordfighting mission. The follow-up to that is a swordfighting mission ('Wicked Communion') that takes place in a fortress. Once completed it can be handed in by returning to the fortress through the Longboat Coxswain.
Handing it in unlocks the daily mission 'Baptism By Fire' as well as another, non-repeatable ship combat and swordfighting mission ('Turnabout Is Fair Play') with one of the good turn-in ship outfittings as a reward.
The reward for completing 'Baptism By Fire' is 5,400 db (not for the first completion though) and 2 turn-in items.
3 turn-in items can be traded in for various good swordfighting or general ship outfittings, 4 turn-in items can be traded in for clothing items, 15 for a title, and 25 turn-in items get you very good swordfighting or general ship outfittings.
There is an additional special reward of 5,400 db and 1 turn-in item for completing the mission with a small group (up to 4 group members).
Another special reward of 5,400 db and 1 turn-in item is awarded for successfully defending both of the coastal villages (one is enough to complete the mission).
The additional rewards are handed out via missions that only require talking to your contact in the fortress.