In a universe where drowning doesn't lead to death, feral cetaceans are sapient, and quite a few people manage to befriend them. However, with how long said cetaceans can hold their breath - longer than anthro ones - staying with them for more than a few minutes isn't possible for many people. Not without scuba gear... Or in this case, a different option for those who prefer holding their breath.

Known as an "oxygen balancer", these devices come as bracelets, anklets, and collars, all of many different sizes. Tapping two balancers together pairs them, and then while both are worn within 100 feet of each other, the oxygen of each wearer is transferred between them as needed - in essence, how long they can hold their breath is averaged between both wearers, plus or minus a little based on willpower as normal (Modifying the size of their breath meters accordingly, if you're using the optional breath meter lore).

In the case of this lizard (A record of 3 minutes, each blue bubble representing 30 seconds and each dot representing a used one as they slowly pop) and this small whale (A record of an hour and 17 minutes), that's given both of them a maximum breath hold of roughly 40 minutes. If a friend with a long breath hold is willing to temporarily give some of it up to someone else, they can stay together much longer!

UPDATE 12/5: Something else I thought of adding. If an oxygen balancer breaks or disconnects while active, the effect immediately ends and both wearers' oxygen usage returns to normal (i.e. if someone who can normally go 3 minutes had been brought up to a half hour limit, then held their breath for 15 minutes before their balancer was broken, they then have a minute and a half left.)