I have a potential scenario coming up where my PCs will be in the sewers under a large city. There will be a Circle of the Moon druid who likes to Wildshape as soon as the party enters anywhere dungeonesque. Since there is going to be water in the sewer they might want to use the chance to shift into a giant octopus (or other aquatic shape).
My Question is: How deep would the water in the sewer need to be to allow the player to use the 60ft swimming speed of the octopus over its 10ft walking?
I'd like to accommodate the use of different wild shapes whilst still having the sewers to be realistic.
I'm mostly looking for specific rulings in RAW, or in the rules as intended by the authors, to ensure fairness to the players and allow for parts where they can and can't swim/breathe.