. . . I don't think that there should be much, if any, limitations on animal associations. Take for example Legend of Zelda's Windfish, Poseidon's creation of horses, the flaming Phoenix, the Thunderbird, or Capricorn, the goat of the sea.
Windfish are okay (they even have real-life precedent in flying fish), but
Firefish would not be. The elements of Air and Fire are not opposed, and Air and Weather are actually allied, so Phoenix & Thunderbird are both good. Capricorn is always depicted with a fish/mermaid tail. (If you want to stick an animal where it doesn't belong, tack on parts from animals that DO belong. Aztecs be all, "You want to put a Serpent in the sky? Better make it Feathered, dude.")
Frankly what comes to mind for me when I hear that an ocean god is riding a Giant Desert Scorpion is that the dwarfs misinterpreted what is actually a lobster.
Unlikely, as dwarves know about scorpions, lobsters, and cave lobsters, and we must assume they can tell the difference.
Your other suggestions seem quite sound, by the way.
I'd say that the interesting thing about this is that you could tie almost any pair of things together with a bit of thought. . . . Gods were often tied to things seem to make no sense at first glance, so why couldn't a mountain goat be tied to a god of the ocean, climbing the many cliffs and valleys of the waves during storms, or a giant desert scorpion to scuttle over the dunes of the calmer waves?
I find your lack of hydrodynamic knowledge disturbing. As for unexpected associations, as I've said, I have no problem with a deity having traits that seem bizarre, as long as those traits do NOT run directly counter to that god's domains. For example, please try to rationalize how a blind cave bear somehow managed to beat out every single species of bird to become the animal associated with a sky god. Or why a god of pregnancy would be represented as a skeletal male dwarf. Or why a god of nightmares and deformity would be named "Luxury Palacejoy the Festival of Dances." Or why a dwarf civ would show their god of law, honor, and justice depicted as a kobold.
Yes, there
are instances where things that generally oppose each other can be seen to have a common link--
e.g., a god whose domains include both fame and rumors could be described as a god of reputation. A deity who presides over birth as well as suicide could be seen as a "cycle of life" god. But these are by far the exception, rather than the rule.