Can you just go to spend the day here and not overnight?

Yes u can go for 1 day but it cost less to go overnight. Tickets for the day r $99.00 per person & I stayed overnight with 3 adults and 3 children in 1 room for only $222.00 water park included & we used it 2 days

Yes, my wife and I spent the day a week ago. You buy what's called a splash pass. You have to buy them online. Depending on the time of year, depends on the cost. Ours was $99.00 per person because it's still summer but a week later was $89.00

I dont think so

Yes you can go for the day but it's cheaper if you stay, they charge 100 or more

Absolutely yes, and you can reserve a Cabana there if you need a private place.