It does seem like I get more than 20 seconds between the cry and the sink but I'm not sitting here with a timer to verify it so it could just be that panic mode makes the time seem longer. At this point I start with the barnacles furthest from my boat, when they cry out I move to the barnacles closer to my boat, and I don't tend to hop off until the actual bubbling starts. I would believe it's 20 seconds from the bubbling to the sinking, but it seems like more than 20 seconds in total.

I've so far gotten enough this game for the harpoon but I'll still need a bit more for the knife. I get that it's rng based and I could just be unlucky or dealing with observers bias, that's part of why I asked the question. I do think at least part of it is my poor luck though. I tend to have weird luck in games, it's amazing when it counts and I survive things i really shouldn't, and horrible when it comes to just general mechanics like spawn rates or the sink in this case. Like this leviathan hunt started as a serpent hunt but I had better luck finding the leviathans since they just sort of pop into existence in front of my boat and I nearly crash into them, but considering I am hunting serpents, sure lets go ahead and get the tool that makes it easier to get their scales compared to kiting them into shallows (even if it's mostly meat I'm after).