The Water Splash is a ride improvement that can be built for the Wooden Roller Coaster. The water splash increases the excitement rating of the coaster, but slows down the train a little bit. It is therefore ideal to build the water splash at (or near) the end of the track before the station, to slow down the train before it enters the station, to help prevent crashes while also boosting the coaster's excitement rating.
The water splash takes up five tiles in a straight line. If you want to build a water splash at 0ft, you must first dig down five squares of land in a straight line, since the track piece dips down 5ft. The water splash can however be built at 5ft or higher with out you needing to lower any land.
In the first game, it can be researched through the "ride improvements" option under the reseach tab. Once researched, it'll appear when building a wooden roller coster, under the "special track elements" tab on the ride costruction menu. However, in the second and third game, the water splash is available by default (since there is no "ride improvements" option in the research menu, all special track elements for all roller coasters are available by deafault).