The Soaked addon adds more roller coasters and carousels; Also swimming pools can now be built. The new features are also in the RCT3 careers available, what makes some scenarios more easily and the impatient players will help to progress quickly.
The package contains two CDs, so you can install without RCT3 also addon. The RCT3 CD is always required to play.
However, anyone who intends to still zuzulegen the Wild addon should rather RCT3 buy individually, because there is the well-ons to buy new only as a bundle (Soaked and Wild). The individual game addon can be found most likely still needed for resellers, but what then can be just as expensive as the new double-bundle.