Brisbane Wheel is just over 50m diameter, and the footing requires around a 25x25m pad
Melbourne Southern Star Observation Wheel is around 110m diameter, footing goes about 55m wide, not sure on the overall footprint though as the supports dont appear to be symmetrical...
The castle space could absolutely accommodate a wheel, though the size would have to be carefully considered as its not super straightforward - the widest clearance (the diameter of the wheel) required is only halfway up the wheel, so you can potentially build over top of some other facilities or attractions. The castle space is also elevated from it's surrounds, so it's already clear of almost every nearby obstacle that you could probably fit a Southern Star sized wheel in easily, giving more spacious cabins than what Brisbane offers. While the footprint is not big enough for what is in Melbourne, the majority of the footprint is open space, with a support column near the corners - this could easily be worked around the existing pathways in Sea World without much impact or redirection.
Personally I think it would be best in an East-West orientation (top to bottom in the picture below), giving views up and down the park for the majority of the ride, rather than primarily looking at water on both sides.
(And if the monorail station is removed, there's even more space - plus those bathrooms underneath it could do with a refresh)