Main construction work will start next month with a 100-strong site team due to finish the scheme by 2013.
Simons beat rival bidders Bowmer & Kirkland, Willmott Dixon, Vinci, Kier, SDC and Kiernan to land the job.
The new waterpark will include new slides, a “lazy river”, wave and whirl pools and a 28m flume tower.
Around 250 tonnes of new steel for the complex will be delivered to the site in June by specialist fabricator BSB.