You can buy lilos, beach balls etc. and even buckets and spades. Take your own suncream though as it can be expensive out there.

The Waterpark (Acqua Palace) is in Port el Kantaoui, just up the road from Sousse. It is open from 9.30 a.m. to 6 p.m. every day from the beginning of May to the end of October: Adults 12.5 dinars, children (under 12) 7.5 dinars and under 3 free.

A taxi from Sousse to PEK should be no more than 4 dinars (less than £2).
If you do a search for Water Park on this forum you will find quite a lot about it. You can also have a look at their own site:

http://www.acquapalace.com/

Hope you have a lovely time. The Royal Beach is actually right on the beach and a nice easy walk into Sousse itself and the Medina.