Underwater World is an aquarium - I've haven't been there myself (I've lived here 9 years and keep meaning to go and check it out)

To stay - when I was a kid (too long ago) we stayed at Alexandra Headlands, Cotton Tree and Maroochydore and they were all good. I'd suggest not going to Coolum this year (even though I live here) 'cause they're about to knock a block of shops down and re-build.