If you just want a cooling off / wet option, there are several opportunities for that within LEGOLAND itself - both rides and the squirty fountain places (just bring extra clothes).
But if you want the Lazy River, water slides, wave pool, and those kind of things, there are the 2 extra waterpark sections.
I'd say with only 1 day, LEGOLAND itself would probably be plenty, unless your kids really love the waterpark stuff.