While it is unfortunate that this leads to most US zoos not having aquariums, wouldn't a separate aquarium facility lead to a greater number of species viewable within a city? Take Los Angeles for example, which has the famous Los Angeles Zoo as well as the Aquarium of the Pacific. In January 2018, I recorded having seen 159 species of fish at the aquarium (with there being several I never identified and surely more I missed completely) but only 4 (plus a few rainbowfish species I think) at the zoo, with zero overlap. When looking at other groups, though, we have 71 mammals and 52 birds at LA vs 3 and 17 at AotP, with only sea lion and Harbor Seal overlapping. The result is 287 mammal, bird, and fish species being represented in the city of LA, whereas your average zoo-aquarium would likely have much fewer of one or more of those groups (most likely fish) as a result of the increased biodiversity.
Additionally, the presence of an aquarium will not automatically mean the number of species is drastically increased. Using two of my favorite zoos as an example, Chester Zoo has an aquarium but it only holds 42 species in it (though yes there are a number of other species held/recently held in Monsoon Forest). Meanwhile the Bronx Zoo keeps roughly 36 (will be more, it's been a while since I've attempted to ID several of the unsigned fish in CGF) fish species despite the lack of an aquarium building. Yes there are fewer species at Bronx overall than are kept in Chester's aquarium, but the difference is not "much lower".
Also I personally think that the claim that the lack of an aquarium usually makes a zoo bad is utterly ridiculous.
~Thylo