You are much better off using a swim AO, you won't then need to swim in fly mode then. They will usually regulate your height in the water with page up/ page down keys.
Note there are AOs for mermaid swimming. Historically tails were Flexi but are now commonly mesh the mesh ones are animated using additional bones so for a mesh tail I would be careful if using an old AO you find on marketplace. Check the description to see what it says, but I would think anything you find from the last 8 years would support mesh tails.