For the Hidropark, you need to get off at the Magic Circle in Alcudia. Which is just the name of a shopping/restaurant mall by the roundabout near the port....just by the go-cart track. Then you need to make a short walk towards the Jupiter/Saturno complex....but you'll probably spot the Hidropark because of the flumes
One word of warning....in summer the buses can get very crowded, and they'll just drive past if they're full up
We've also been on buses where a group of about 20 Spanish kids is being escorted to the beach.....they're incredibly well-behaved, but take up a lot of space!