Why does the poet suggest that the dolphin and the nereid should have helped the favorite?

Selima, a tabby cat, is pensively watching two beautiful fish swimming around in their aquarium. They are so lovely and magical in appearance that the speaker actually compares them to angels or genii. The cat cannot seem to help herself, as she is so drawn, so attracted to the beautiful fish that she must reach for them. What cat, the speaker asks, turns away from a fish?
The cat stretches out to reach into the water, and she unbalances herself, stretching too far. She falls into the water. The speaker describes her frantic meowing to "every watery god"; in other words, Selima cries, in panic, for anyone to come to her aid. I'm not sure the poet suggests that a dolphin or nereid ought to have come to save her, but there are many allusions to mythology in this poem—to Fate, to nymphs, to "water god[s]"—and this makes what happens to Selima seem almost mythic, or fated.
Unlike in the myths, however, there is no magical savior who comes to rescue her. Tragically, nothing came to save her, not even creatures associated with the water or "Tom" and "Susan" (who, I presume, are Selima's owners).