True, but not all marine mammals do (for example fur seals and bottlenose dolphins stay relatively shallow)
Actually not exactly true . Most cases of the bends inhumans arise from breathing compressed air on scuba or getting in an aeroplane after freediving. If you’re not breathing compressed air you can shoot up and down as fast as your airways can equalise and have little chance of getting the bends. (Small risk that increases with depth and frequency of dives, but is considered very uncommon on breathhold dives.) As an example, when I’ve been playing around, I’ve done round trips down to 30m and back to the surface in less than a minute multiple times in a day just for fun because I can (not recommending everyone goes out and side this though because it depends on your ability to equalise!) with no ill effects. No way you’d try that on scuba, you’d almost certainly end up bent.
That is true, but tbh, humans probably wouldn’t hunt like that anyway unless driving fish into a trap. It’s reasonably fast and efficient for travel, but not great for hunting. Humans would do better either as underwater gatherers (IE plants, mussels, crabs etc) or as ambush predators.
Because what has made humans most successful is their intelligence, ability to work in groups, lack of specialisation and their ability to use tools. Losing the use of hands, severely impacts the last one. Humans on land are also terribly adapted from a straight survival POV save that we are smart and generalists. If we couldn’t use tools and adapt, we’d have gone extinct long ago, but instead we’re the dominant species in the planet.
(I can’t believe I’m debating mermaid physiology lol)