Humphrey’s argument fish aren’t conscious doesn’t only rest on their not having the requisite brain structures, because as you say, it is possible consciousness could have developed in their own structures in ways that are simply distinct from our own. But then, Humphrey would ask, if they have visual sensations, why are they uninterested in play? When you have sensations, play can teach you a lot about your own sensory processes and subsequently use what you’ve learned to leverage your visual sensations to accomplish objectives. It seems odd that an organism that can learn (as almost all can) would evolve visual sensations but not a propensity to play in a way that helps them to learn about those sensations.
Perhaps fish just don’t benefit from learning more about their visual sensations. The sensations are adaptive, but learning about them confers no additional adaptive advantage. That seems a stretch to me, because it’s hard for me to imagine sensations being adaptive without learning and experimenting with them conferring additional advantage.
You could also respond by citing examples where fish can play, and are motivated to sensation-seek, as you already have, and I think if Humphrey believes your examples he would find that persuasive evidence about those organisms consciousness.
To me “conscious pleasure” without conscious sensation almost sounds like “the sound of one hand clapping”. Can you have pure joy unconnected to a particular sensation? Maybe, but I’m sceptical. First, the closest I can imagine is calm joyful moments during meditation, or drug-induced euphoria, but in both cases I think it’s at least plausible there are associated sensations. Second, to me, even the purest moments of simple joy seem to be sensations in themselves, and I don’t know if there’s any conscious experience without sensations.
Humphrey theorises that the evolutionary impulse for conscious sensations includes (1) the development of a sense of self (2) which in turn allows for a sense of other, and theory of mind. He thinks that mere unconscious perception can’t be reasoned about or used to model others because, being unconscious, it is inaccessible by the global workspace for that kind of use. in contrast, conscious sensations are accessible in the global workspace and can be used to imagine the past, future, or what others are experiencing. The cognitive and sensory empathy that allows can enable an organism to behave socially, to engage in deceit or control, to more effectively care for another, to anticipate what a predator can and can’t see, etc.
I would add that conscious sensation allows for more abstract processing of sensations, which enables tool use and other complex planning like long term planning in order to get the future self more pleasurable sensations. Humphrey doesn’t talk about that much, perhaps because it’s only a small subset of conscious species that have been observed doing those things, so perhaps mere consciousness isn’t sufficient to engage in them (some would argue you need language to do good long term planning and complex abstraction).
Humphrey believes that mammals in general do engage in play, which he thinks all (but not only) conscious animals do, and that they also engage in sensation-seeking (e.g. sliding down slopes or moving fast through the air for no reason), which he thinks only (but not all) conscious animals do. And he’d say the same thing about birds, and the fact that those behaviors’ distribution over species lines up nicely with the species with neural structures he thinks generates consciousness he treats as additional confirmation of his theory.
Animals do engage in play with unpleasant experiences, e.g., playfighting can include moderately unpleasant sensations. I suppose the benefits of those experiences being conscious might be to form more sophisticated strategies of avoiding them in future. It isn’t that Humphrey thinks play is necessary for conscious to emerge, it’s that he thinks all conscious animals are motivated to engage in play.
I feel this last answer maybe hasn’t answered all your questions but I was a bit confused by your last paragraph, which might have arisen out of an understandable misunderstanding of the claim about consciousness and play.