This might not be an investment into the park itself, but I'd consider any investment a positive myself! Given that Jumanji is opening in 2023, we surely can't expect a new major investment into the park itself for a good few years yet, no? Jumanji should surely be good enough to tide them over for a good few years yet, and I think it's great that Merlin are building them such an expensive new coaster and land in the first place!
Besides, an indoor waterpark is not an insignificant investment, and given that there are currently slim pickings waterpark-wise in the South East, I'd argue that it's also quite a shrewd one from a business perspective. As I said, there are few waterparks in the area, and the indoor factor will also give Chessington a second gate attraction with good year-round and all-weather appeal. Furthermore, a waterpark would considerably bolster Chessington's credentials as a family short break resort. It could have a lot of potential in this area given the considerable population in its immediate catchment area and lack of nearby competition! (While LLW has nice-looking hotels, it doesn't really have any second gate activities as such, and none of the other parks in the South East have an overly substantial, if any, accommodation and resort offering for families)
I should also note that Jared, the administrator on Chessington Buzz (who first dropped hints about an indoor waterpark a few months back), has suggested that there may be multiple attractions discussed in this consultation rather than just the one individual attraction, so the park itself may well see an additional investment to supplement this after all. Take that with as much salt as you like, but given CB's excellent track record for accurate rumours, I trust Jared implicitly.