Following my query about zoos (I'm a bit ambivalent) why not one about aquaria? My youngest son has always liked these - so if we're in any town with an aquarium then we have to visit. I must admit that I find them a bit tedious after a while but he could spend all day in one - he also spends all day looking in rock pools ....
I've not really seen a bad one but the two we liked most were the newish one in Barcelona - big and beautifully laid out although some of the labelling was a bit iffy. A real favourite is a small 'old style' one in Banyuls-sur-Mer, south western France (which is, incidentally, a good place for exploring the coast and eastern Pyrenees -
http://www.banyuls-sur-mer.com/anglais/c1.html ) attached to the oceanographic study department of the Sorbonne, as I recall. Not vast amounts in there but some oddities and well presented.