Deployments of weeks or months to map the seafloor, survey cables, measure pH, salinity, and temperature, and perform other missions.
This Phish with "medial paired fins" will have a battery pack which runs its length and would have the longest deployment time. A 20' Phish like this would have a deployment time measured in weeks to months.
The large central opening would pass through a hollow drive-shaft, which does not penetrate the isolation chamber. This would exhaust heat, provide a quiet place for sensors, and provide a connection place to recharge batteries. Fins could look different than this, though these would be reversible with a flooded, low angle actuator, making the Phish highly maneuverable.
Proof-of-concept video
This was an early version of the Phish guts which didn't have a lot of power, but it shows the idea.