This fleet is designed simply to have a pair of ships that exceed in their role, despite being relatively unorthodox choices for each. Coleoptera's not the greatest submarine, and in a world shared with ships like Hai Peng and Banshee's Cry, Raven's Neck isn't the greatest treasure runner, but some choice crew make these two into potent ships nonetheless.
Coleoptera is the warship of the two. With the Helmsman, her move is a rapid S+S+S, and she can roll for a repeat of that move every turn, thanks to SAT Calico Cat. Paired with the re-roller Lady Baptiste, this gives her a 55% chance at an extra action each turn, making her capable (and likely) of being capable of a single-turn movement of S+S+S+S+S+S. Her purpose is to repeatedly ram while submerged, gradually damaging ships with minimal risk of retaliation. The fact that she's submerged will make it difficult for enemies to get at the Hag.
Raven's Neck is the gold runner. She's fast enough, at L+S, and her hold space of 4 means she can take a whole island's spoils... and she'll know which one to target, too, because Devereaux and her own keyword will have revealed that to you. She can spy upon enemy gold runners as well, so Coleoptera can capitalize on the knowledge and target high-value ships first. She can also use her ability to avoid the sole UT in play, which is meant to potentially disable enemy gold runners, and provide a more favorable target for Coleoptera.
A combination of high speed, strategic knowledge, and a convenient UT should make this a formidable fleet to play against. Of course, counters do exist. Cancelers and anti-submarine abilities stand a serious chance of making Coleoptera surface, where she's less effective. Raven's Neck stands a good chance of being caught by fast gunships, though any interceptors will have to deal with both Coleoptera and the reasonable armament carried by Raven's Neck.