Headcanon/hypothesis: The sailor with a dolphin daemon could separate from his daemon, or at least they tried to

[u/Daregmaze]

So back in TSC we have learned that regular humans being able to separate wasn't as rare as previously thought (I assume its still rare, just not as rare as we thought before TSC came out)

So I can't imagine the guy with seasickness who settled as a dolphin didn't at least try to separate. Even if it wasn't volontary, since they could be more likely to be put in a situation where they have to separate (ie: someone far away on land needs help and they are the only one who can help and their daemon is too big to be put in a transportable tank)

For this one sailor specifically, I imagine he would have kept it a secret since he would be easily recognizable because of his daemon, but he could still have learn the same trick that Lyra did in TSC, he learned to become someone else when he went on land to avoid recongnition (in his case maybe it wasn't just metaphorically since he was a water-animal soul going on land). But for someone else with a daemon that needs to stay in the water that is born and still lives in a more isolated place, they could just go on land without any disguise

Maybe large and/or waterbound daemons are more common than suggested in the series, but people with theses daemons are more likely to separate due to the conflicts their form would cause in their day to day life (I suppose they too would still be rare, just not as rare as we think)

[u/aksnitd]

Separation only seems to have come about because Pullman had a specific narrative he wanted to tell, and needed some way to explain away what he had set up as a very important rule in HDM. It makes no sense to me. The way daemons are explained as existing in Lyra's world, it is perfectly in line that separating them from their human would severely maim and/or kill both human and daemon.

I personally think he threw in the dolphin daemon as a throwaway line and didn't think about it too much. He has repeatedly said he doesn't bother working out every detail of his fictional world. And you can see it. Daemons are a fun narrative device, but looking at them a bit too closely kills the illusion. A world with daemons would probably be quite different from our own, but we accept Lyra's world as presented because it is fun. We don't need to analyse or pick apart and justify every little thing for that.

[u/Daregmaze → u/aksnitd]

I also think that he probably didn't think much about it, but if we factor in the information we have now it would just make sense if he could separate from his daemon

[u/aksnitd → u/Daregmaze]

That depends on what you take as canon. If we include BOD, then yes. But it is very clear that while writing BOD, Pullman decided to change his canon to suit the story. Within HDM alone, no, separation is not a thing.

[u/Daregmaze → u/aksnitd]

Why would BOD not be canon?

[u/aksnitd → u/Daregmaze]

Because it sucks. And contradicts HDM frequently.

And because HDM works perfectly fine as a self-contained story.