Also, do you get to access to the park the day you arrive and the day you check out? We are only staying a night!

Yes, both days! 1 pm the day of check in and all day the next day, but you do have to be out of the room by check out time.

Yes you can swim the day you check in and the day you check out. Which is great considering it's $110 for one day and one person

Yes

Yes indeed! There are showers and changing areas right in the waterpark.