Went to Menorca last year and can confirm that the 2 waterparks were still open then. We didn't visit so can't answer your other questions I'm afraid.

Yes both water parks are open, and yes they are cheaper if you go in the afternoon.

I personally would say just go for the afternoon and make some savings. Both waterparks a quite small compared to other bigger ones on the mainland, so you dont really need a whole day to go round the parks!!

My personal favorite is the one in Calan Forcat, but there really isnt that much difference.

For the waterpark in Cala'n Forcat, does anyone know what the prices are reduced to later in the day, and from what time they apply from (is it 3pm)?

Full day prices, for anyone who is interested by the way, are:

10:30-18:30

Children 0-3 Free
Children 3-11 (10 Euros)
Over 11 years (17 Euros)

EDIT: Just to answer my own question (back home 2 days now (7 August)), after 3pm it's not quite half price. It's 11 Euros instead of 17 for adults, and 7 Euros instead of 10 for children. The park closes shortly after 6pm, but we found 3 hours was plenty for our two (6 and 3).