Breath of the Wild
The Water Reservoir is located just southwest of Hyrule Castle, south of the Moat Bridge, between the Quarry Ruins and West Castle Town. There are some Hyrule Bass and Staminoka Bass swimming around in the water. While Link is navigating the area, he might be spotted by the dangerous Guardian Stalker found just to the east at West Castle Town.
There are a pair of treasure chests found in the reservoir. At the south end there is a chest underwater that contains a Silver Rupee. To the north there is a chest that is underneath the slab. Use Stasis to move the slab and bring the chest to get five Bomb Arrows.
Nearby Korok Seeds
There is a spiral of rocks. Grab the rock nearby and set it in the center to complete.
Tears of the Kingdom
The Water Reservoir is part of West Castle Town, located just north of Lookout Landing. The reservoir has numerous Hyrule Bass, as well as some less common Staminoka Bass. A treasure chest can be found in the water which contains a Blue Rupee.