Can you buy memberships at the park?

Yes you can buy memberships at thr park

Just take care that the water park in the summer closes at 6:30 so if you only have one day go there first then to the theme park.

Yes. But if you buy online or by phone before and get parking pass too, then you will save that high cost too.

You can buy a single day ticket and upgrade it to a season pass on your way out

Yes

Yes, you can.

Yes it's in there website or at the desk in front of the park.