Not for aquatic aliens in the current state of the game, yeah. But portrait packs limited to certain types of worlds no one else hass access to, why not.Stop suggesting gameplay effects or restrictions for portraits.
Like an energy/gas beings pack linked to gas giants, for example. That's something I could see in the far future of Stellaris.
And even for aquatic aliens : if they are definitely aquatic (that means, they are swimming in the water in their diplomatic rooms, and their cities in the background are clearly underwater), there's no reason to be so strongly opposed to their existence. It would only bring more diversity among the portraits.
Aliens stuck on a certain type of worlds unless they terraform them is not per se a bad idea, and you'll need arguments to prove otherwise.