I, too, love a waterpark and amusement park thrill ride as much as the next Roller Coaster Tycoon alum. However, the pragmatic adult I've matured into wants to point out the immediate concerns that arise with what you're proposing.
The waterslides/waterpark would be a maintenance nightmare... not only for the elevated facility itself, but especially for the Central Artery infrastructure beneath the surface. Water penetration from the surface has a long-term impact on the structural integrity of the Central Artery and ramps. Adding pools and water slides above these will further accelerate the rate at which MassDOT needs to invest
literally hundreds of millions of dollars to maintain the I-93 tunnel. On top of these concerns, the facility would not be usable for nearly half the year due to frigid temperatures. You can make an argument about an enclosed waterpark facility and enclosed water slides a la Great Wolf Lodge or Jay Peak Resort working in a winter climate, but the maintenance costs to operate all seasons in such a facility most likely far outweigh the market rate families are able/willing to spend to go to a water park.
Similar to the waterslide idea, I think a roller coaster's not viable for about half the year. I use to fantasize about a wooden or launching roller coaster that would operate from North Station to South Station like a shuttle train... but such a facility would decimate the natural environment that's evolved the length of the Greenway... not only with shadows and/or light pollution, but particularly noise pollution. The screams from roller coaster riders would not make great neighbors to multimillion dollar downtown condo owners (et al).
In a downtown rife with built environment, the most special new developments are ones that enhance/increase the natural environment and our access to it. I would welcome something like New York's recently opened
Little Island elevated over the ramps, and providing parcel connectivity via pedestrian bridges over some of the cross streets like N Washington, Sudbury, and potentially Hanover/North Streets. We should be reevaluating opportunities among our vehicular infrastructure to create more places for people to just be.