At the risk of repeating a conversation
@jon81uk and I had a few
months years ago, my personal view is that Merlin (for now) see Cariba very much a supplementary part of the resort and not a key draw, whereas the Center Parcs waterparks are very much integral to what they offer (I wouldn't visit if it was closed - would you?). I broadly agree this is the correct way to position it, and certainly so before Alton really ramp up their efforts to be a year-round resort.
This position may change and we may see further investment in the waterpark, particularly with the focus on accommodation that is planned, the easiest way to get year-round revenue up on that front is to better exploit the waterpark and I think that's a tough sell with the current scale of the offering.
Great Wolf, Kalahari etc have done a hotel/waterpark hybrid with some success in the US market but they are very different businesses. I think Alton's biggest battle is convincing people to come out to the resort when the major coasters are closed, or booking to come weeks/months advance when some signature attractions are open but that it could be a total washout. Having a large-scale waterpark to fall back on could be a powerful marketing tool, but there are downsides of doing that too.
A day visit to a waterpark is probably going to be satisfied by Water World, Sandcastle or similar for the local market and other more distant metro areas have something equivalent to Cariba not too far away. For Merlin, I suspect the day visit waterpark market is not something that they are interested in chasing - people typically turn up, spend little more than the gate price and leave. Plus, it makes the facility far less exclusive for resort guests, which is a key selling point for them.