If I bought a San Diego Go card from Costco can we do legoland and the water park in the same day

You can do both, but make sure you get that at a good time if you are doing both

You can do both, check when water park opens

I think so

The water park wasn't open last week...

My wife purchased our tickets from Costco and we used them to gain free access to the water park, which is within Legoland. You can do both in the same day.