Your ticket options will be very limited since you waited so long. Since you are staying off site and that rules out the UPH, the only way you can do what you propose is by getting a Park Hopper Plus (PHP) ticket. Park Hopper (PH) tickets are for the major parks only (MK,EC,AK,MGM) while the PHP also includes a limited number of admissions for the water parks. The number of those admissions depends on the ticket you buy.
You will pay more for this ticket by purchasing it at Disney instead of in advance. You will also have to get the 5 day PHP (has two plus options) which is the smallest they sell to the public at the gate.
What I would suggest now is that you visit one of the ticket wholesalers when you get to FL and get your tickets in person from them. Maple Leaf, Ticketmania, OTC and several others have offices there where you can pick up PHPs for less money than you will pay at Disney even if you had advance purchased them. The wholesalers can offer you a 4 day PHP (which has one plus option) which you cannot get at the Disney gate.