Weather shoudl be fantastic .......... hot but not too hot
Waterparks will be fine and I doubt you will need anything more than sleeveless in the evenings
I guess there is more chance of a "bad spell2 in May/June than in July , but only a little bit more
When we still had primary school hols which included 2 weeks in May/June , we always came those weeks( 7 years on the run ) and I can only rememver one year with several days of awful weather ....... other years may have had the odd overcast or rainy day , but generally hot and sunny