The Water Planet is a scrapped location in Serious Sam: The First Encounter.
The Water Planet is presumably a planet with water everywhere, or at least as a large part of level design. The planet's Elemental enemy is the Water Elemental. It appears to have been the largest out of all the planets the player was going to explore after leaving Ancient Egypt, as it has four levels; EndLakeMountains, IntroandWaterfallCircle, TerraAndRiver, and WaterfallValleyAndStairs. The unusual level order (the end area seems to be before the intro area) is likely to help with beta testing.
The player would be sent to the Water Planet after completing the last Ancient Egyptian level; PyramidValley (not The Great Pyramid, which is the final Egypt level in TFE). After going through the Water Planet, the player would be sent to the Lava Planet.
Like all of the post-Egypt story arc locations, the Water Planet was scrapped before the final game was released. However, unlike the other scrapped planets, it seems that a part of it might still be in the final game in the form of Moon Mountains. One of the Water Planet's levels is named IntroandWaterfallCircle, which matches the large waterfall circle in the middle of Moon Mountains. Based on this, it seems very possible that IntroandWaterfallCircle was reworked into Moon Mountains when the Water Planet was scrapped, as Test 1's co-op level order doesn't show Moon Mountains, but does show IntroandWaterfallCircle.