At the cage boss I ate all of his minions and grew rather large. On the next level the game started "zone cleared" after I ate just one enemy, and on the level after that "zone cleared" sequence started immediately, spawning the very first whirlpool right at my position. And I noticed that whirlpools only show the DIVE button when you move into them from outside. I was curious what will happen if I just wait in the whirlpool... In the old days it would just force me to dive, but thankfully these days are over. However, it just kept growing and then other whirlpools appeared and grew bigger, and then I thought "hey, how am I gonna dive without DIVE button? I need to enter it from outside, but it's whirlpool everywhere and I don't have my button..."
In other words, welcome to the second circle of Dante's Inferno.
Eventually I managed to find the last small patch of still water and the DIVE button appeared as I entered whirlpools again, but that was lucky.