Tetrapod,
Whilst I agree that the Singapore situation is perhaps an example of how NOT to do something, there are other examples of fish with seals and penguins that work well and create a great display.
You don't need to keep seals, dolphins and penguins in chemically-treated water if your facility is seaside. Good husbandry and thinking outside the square can get you far.
Sydney Aquarium keeps Little Penguins with bream, trevally, Port Jackson sharks, Fiddler rays and large sand whiting with little stress evident to either the fish or the birds. The pool also has aquatic vegetation such as kelp, invertebrates like snails and anenomes, and a dump bucket for a tidal surge, creating a dynamic and naturalistic display.
Taking size and species into consideration for a mixed species display is crucial. The above exhibit has fish that are too large to be food for the penguins, along with large social groups of both, which are well fed. The same appears to have been true for the seal exhibit at the Aquarium. Hula fish are too small and uninteresting for the seals, there are plenty of nooks and crannies for the schools of sweep etc to mooch around, and the seals had other stimulation. No doubt like for your normal oceanarium with sharks and fish there are some prey item casualities, but basic management of the display and trialling different species can and does work. Blood baths are in no one's best interests, and need not be the outcome for these display inhabitants.