Lanzarote's only waterpark is in Costa Teguise - take the main road out of town towards the golf course and you'll see the park on your left.
There are three main slides, of varying steepness, which plunge into the main pool area, and there's a terrific "rapids" ride which you do on a large rubber ring and which takes you all around the park. A new ride is a kind of bowl into which you're shot from a tube and you can almost "Surf" your way down to the exit - great fun!
For people who don't want to spend all their time in the water, there are plenty of other attractions, including bouncy castles, kid's rides, trampoline and a bar and restaurant. Adults can relax on comfortable sun beds around the main swimming pool.
Plan a full day at the aquapark, as entry isn't cheap at €22.50 for adults and €17 for children. There are discounts available for residents. Also make sure you pick decent weather if you're going in winter as the pools aren't heated. The park is open from 1000 until 1800 and normally closes for a couple of months during winter.