Big thanks to OP for starting this thread; it's been a lifesaver this week.
Here's my experience:
The water shoes at the park weren't that expensive (€9.99) so if you get there and find yours don't fit, don't worry about it and just get a new pair. It's worth it. That said, DS did not bother to wear his much of the day and there were some piggy backs needed to go up the hills.
I did 19000 steps that day, most of it uphill, some of it with a child onboard, and that's not including the time I dozed off for half an hour when they went in the wave pool. The fast pass is worth it for not queuing but it means that you will climb the stairs with no breaks like the people queuing have. Stamina a must.
We decided not to do a second day although that was the original plan - the kids age 8 and 10 found it hard going, especially around lunchtime. It took "one last ride" at 2-3pm to inject a much needed adrenaline dose to get enthusiasm going again and then we stayed til the last wave series.
The wave pool has 10 waves 2 min apart starting at 20 minutes to the hour, and then ceases on the hour for 40 minutes. If your kids enjoy that, you can use it to time your day ("we have 30 minutes til the next lot of waves; let's do The Dragon and come back") otherwise you'll probably find yourself spending your whole day near the wave pool.
The Tower of Terror is over 14s only. DD8 was devastated not to be allowed on it. You get to not give it any headspace. Whew!
Similarly I saw lots of too-small kids being pulled out of line at the top of the queues to be re-measured and sent back down; the staff are pretty safety-conscious so if it says minimum height 125cm don't waste an hour chancing it in the queue because they will check.
The food was really good, much to my surprise- DD hoovered up the pizza, DS10 ate every scrap of the Thai chicken curry, DH said the burger was good, and I had a salad which was reasonably priced and all had plenty when we shared some chips too.
The slides are RAG rated for difficulty, which means the red ones likely have a drop, but not more than one so if like me you are only going on these things to humour your children then just keep your eyes shut and you'll be fine.
We got horribly sunburned. It feels colder in the water than it really is, and the May half term is so early in the season no one has any tolerance built up. Suncream. Rash vests. Shade. Hats.