While cycling home in pouring rain trying to balance my flippers on one handlebar and drysuit in the other, I pondered what underwater mobility aids would look like for merms. One very handy dandy alternative is those underwater scooters, just with more power and maneuverability, probably longer and countoured to fit a person's upper body well so that you can rest on top of them instead of hanging on for dear life with your arms outstretched. Could also be like those sittable scooters with the air bubble, however, I believe I've made my stance clear on essentially glorified unlicensed scuba divers changing depths frantically
Another option which helps with underwater mobility and doesn't need electricity could be like those aids people give fish with buoyancy issues? Of course, a merm traverses through much larger depth differences than aquarium goldfish — which would mean that eventually, the bubbles on the aids would pop or shrink to basically be meaningless — but I think magic could help with keeping them functional! With an abra cadabra, the bubbles can change their shape and size, pressurising some of the gas to give the bubbles less buoyancy. Using the bubbles would reduce the strain of having to go up and down or maintain your position for merms with some lower body mobility, one can also aid with their arms — oh oh oh flippers! Flippers for arms.... perhaps attached to the forearm to free the hand itself?
With a lot of money and/or practice one could hypothetically create water currents that help them move better, but that also runs the risk of disrupting everyone and everything around oneself, also. Maybe more to come?
To not get too clunky with the design, the bubbles themselves would be the thing to hold the pressurised air; they could be made of a hard material that can sustain the strain of pressure, and *with the use of magic they can shrink and close off parts of themselves or inflate and open up more space, depending on the lift required for a certain depth. The gas could also hypothetically be pushed into the ring around the body, which is a smaller density and therefore the air would still become pressurised and lose lift when needed. Basically, it’s just the same amount of gas circulating!
I’m like 10 mins away from going to bed so I won’t make this digital but look…. Another alternative……. propel with your hands :)
I know it's not necessarily congruent with your other world building but it would be interesting to see a merperson who perhaps have cultivated algae (I can't find it now but I've heard of some animal or fish living symbiotically with plants?) Where they might be able to blow up the air sacks/release the air as needed for their depth. This would of course require they have some sort of control over the release of oxygen here
