Sea or pool?
There aren't many beaches on Malta, but there are lots of ladders that enter the sea.
Mehlleia (sp - see the link below) is the biggest beach on Malta andhas a shallow roped off area.
www.visitmalta.com/en/beaches-and-bays
A boat trip to the Blue Lagoon on Comino is really worthwhile - it's beautiful.
Google for waterparks - there are some, but I've never been.