I tried to save an ask in a draft and instead tumblr ate it so: in response to “how to ride a whale/dolphin,” you basically need a tiny submarine. The rider sits in a streamlined, pressurized capsule near the blowhole, and has a hatch/fan to let in fresh air whenever the mount surfaces to breathe. There are oxygen canisters for backup/safety reasons, and the “reins” are some sort of corded mechanism that lets you tug gently on the pectoral fins.
This (like all of my unusual mount art lmao) is a fairly hasty design and could use improvements, especially with how cramped the capsule is, but I think I’ve gotten the essentials down. You could also ride without a capsule in a scuba suit, but it would greatly increase drag while swimming.