We were there last week. Its much shorter if you dont walk down to the bottom of the hill, but just cut through to your left after crossing the bridge and keep walking - you will eventually come out on the road that the waterpark is on. Yes its definately walkable but last week it was 41 degrees and we had less than half your distance to walk - so make sure you bring your bottles of water for the journey!
We had two 14 year old boys and they enjoyed it, enough "big" slides to keep them interested but they definately compared it to the massive one in Arenal a bit unfavourably.
The advantage of the one in Alcudia is that there were no queues so they were able to slide to their hearts content, the disadvantage being that the slides werent as exciting as the other one.
If you just want an afternoon out its fine though.