Hey there @Etyuhibosecyu! Welcome back to the community! Unfortunately the landscape + water systems are quite heavy during initial operations and will often have two types of issues. One of them being at sufficient scales will use high amounts of VRAM use due to the water system having to access the entire landscape at once and cause the error you received before. This can be mitigated by working with smaller landscapes altogether, and by lowering rendering overhead as much as possible before placing in the water systems. This may be qualified as a bug, but it’s more a limitation to how the base landscape and water system interact, which I believe stems from the landscape brushes needing access to the whole landscape in memory at once.
The second issue stems from landscapes being too large (or mispositioned), often in the Z axis causing issues with how the landscape brush interacts, so often you end up with some water bodies not affecting the landscape properly while others seem mostly fine. Rivers suffer the worst in this case.