During the "interval" at the start of Mission 14, re-enter the Ghost Ship and swim all the way back to the Captain's cabin, entering through the vent. Examine the skeleton of the Captain to start the mission.
Dante is placed in a strange underwater void with eight Blades, and must defeat all of them for a Blue Orb Fragment. The area is large, though not quite as big as it appears: there is an invisible perimeter wall.
The most important piece of advice for this mission is to keep moving: while the pack of Blades initially start out together, they tend to separate into 3-4 groups quite quickly. This decreases how focused their firepower is, though it also means they attack from multiple angles.
It is best to stay at a distance from the majority of the pack to give more time to dodge their attacks. Always assume that one of them is firing at Dante and only stop when there is a clear target, using strafing (+direction, to swim faster) to evade attacks. It is also worth remembering that the Needlegun allows Dante to shoot down a Blade's projectiles, which can be used to blow through their attacks and hit them while they are stationary.