I want to visit the Oceanográfico this summer but wondered what the crowds are like, had anyone been and do you have to queue for ages?

Go on a good, hot beach day and then at least the sun-worshippers will be out of the way, other than that it's a pretty big place so I would think there'd be plenty of room.

The dolphin show is absolutely top notch. I would suggest trying to see a show as soon as you can upon entry as it likely gets busier towards the end of people's visits.

Is it well reputed for its animal welfare standards, does anyone know?

It's like to know about that too - I was just assuming that because it's quite new they would have to be up to standard.

The dolphins look very well cared for. When we were there one of the dolphins had bumped his nose or something so at the end while all the others were playing he was at the side being tended to by his trainer. It was touching to see the strength of bond and trust between human and animal.
Can recommend the show, definitely. (They've also got a group of guys that do high dives into the pool during the show!)

I saw the Dolphin show a few years ago and thought it was very good. But have since watched a documentary called The Cove, it is about the selling of the Dolphins for the entertainment industry and the slaughter of the ones that where not sold. It is one of the saddest things i have ever seen.