Hi Sharon,
Aquopolis is in La Pineda, which is a short bus ride from Salou. You can get the bus from the front, or Plaza Europa in the middle of town. Plana (the bus company) have reps at the bus stops who can tell you which bus you need.
Aquopolis does have dolphins, and 2 years ago you could pay extra to swim with them. The water park itself was a bit old and tatty, and the queues were horrendous (Aug 06), but the rides are ok as long as you're over 1m20 in height.
We thought the water park at PortAventura was much better. It's newer, cleaner and smaller kids can go on more of the rides. There's also a lazy river, which is great fun. The food was better too.
Don't forget PortAventura itself. No trip to Salou is complete without a couple of days there. I don't know if you went to Terra Mitica when you were in Benidorm, but people who've been to both parks tell me that PortAventura is better.
HB