I'm going to Legoland for the first time this fall so I'm not much help but I think it might help people answer you if they knew what time of year you will be going.
If it's summer maybe you would want a break to cool down in the hot afternoon at the water park? Hopefully someone more knowledgeable than I am can help you.
Have been to LEGOLAND several times and we have never done the water park. I don't want to haul around suits and be cold. It looks very cute though. We finish up everything we want to see in the afternoon at Lego so if you want to fill your whole day then add the waterpark or aquarium.
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.