A fish out of water - not necessarily a bad thing
I was thinking about how the fish/fishing metaphor fits in the classic water theme related to Cas. In fact Cas has been associated to fishing even back in season 5 when, after using the angel banishing sigil on himself, he found himself on a shrimping boat, upsetting the sailors (I googled “shrimping boat” to see what one looks like compared to normal fishing boats, and for some reason one of the first results was the urbandictionary entry for shrimp boating. I wonder if google tailors those results to my browsing history or anyone gets that link so up in the results when googling shrimping boat. Anyway.)
Cas is often associated to riversides and in general places where ground meets water: he’s on the shoreline when he gets told not to step on the fish, he gets into the water in 7x01 and gets found by Daphne on the ground next to the river, he is found by Dean and Benny close to the river in Purgatory, he talks with the rogue angels at the beginning of season 10 near a river… and of course there’s the whole thing of Destiel and docks. Better meta writers than me have written about the connection between Cas and water, including sinks and washing (I don’t know if anyone has written something specifically on Cas and bathrooms, from the times he appears to Dean while Dean is washing his teeth or face to his use of showers and toilets when he’s human, etc). The general gist of it is that it represents Cas being in some middle ground between angelness and humanity, both in the sense of his identity and his relationship with the angels and Dean.
If humanity comes from the fish that went out of the water and settled on ground, we could say that the water represents non-humanity (angelness in Cas’ case) while the ground represents humanity. His stepping inside the water in 7x01 represents leaving Dean; he’s keeping away from Dean when he’s by the river in Purgatory, then he gets away from the river when he goes with Dean; the episode in which Dean finds out that Cas is possessed by Lucifer is about a submarine, which is something that doesn’t go on land, and so on.
So it’s interesting to see a fishing imagery applied to Cas… on solid ground. Ramiel, the demon who likes to fish, pierces him with the lance in a way that visually recalls spearfishing, yet it happens on a road. In fact, Cas gets temporarily saved by Mary driving a van against Ramiel, which emphasizes further that everything is taking place on very solid ground.
Basically, just like the fish left the sea and settled on ground and evolved into humanity, Cas is farther away from water (non-humanity/angelness) than ever. He’s stably on ground (humanity). And while he’s still a fish (an angel), he’s on ground now, i.e. human in all the way that count. He loves his human family and his human family love him, he feels emotions like a human (he said it himself that angels and demons feel emotions differently than humans, but both he and Crowley have been feeling emotions like humans for quite a while now…) and he understands humanity (urinating standing for being human since Cas was a human himself).
Just like the primordial fish became humanity by stepping out of the water, Cas is becoming human by metaphorically distancing himself from the water. So he’s literally a fish out of water, but it’s not a bad thing. From the angels’ point of view, well, it is a very bad thing. But Cas has embraced it.
Now, if you ignore the angels who are dicks anyway, there’s one person left that needs to convince himself that Cas becoming human is a good thing - Dean. He feels guilty because he thinks he’s broken Cas; when Cas says they have changed him and he loves them, Dean is pained because he perceives that as something bad he’s done to Cas. (The angels – they don’t care. I think maybe they just don’t have the equipment to care. Seems like when they try, it just… breaks them apart.)