Our toddler LOVES the aquarium. It's included with the park pass and there are fantastic view points for creatures you could not really get that vantage point of in the wild. There are also great interactive experiences if thrill rides are not your cup of tea or if the sand pit, carousel, and silly Duplo train is all there is for you if you happended to be under 36 inches (strictly enforced). Oh, now also the submarine ride for toddlers if you can stomach the 1 hour waiting time. Either way, fun to be had for all. Also, don't forget; annual pass members can now bring a friend for $20... As for water park - there are long queues in the Summer months over weekends. Consider alternate days if at all possible.
Legoland has 3 separate parks. the main park with the rides, waterpark, and Sea Life Aquarium. You can purchase the ticket based on what you want. However, I'm not 100% sure whether you can just purchase the waterpark access. As Isaac Nam indicated above, Aquarium isn't worth the money.
The water park is an additional charge. Roller coasters are included with park admission.
Tickets include all rides but there are separate tickets for Legoland, Waterpark, and the Sea Life Aquarium. You can also get combined two or three park tickets. IMO Aquarium is a waste of money.
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