All but the smallest garden pools require:
An alternative to separate overflows and floor drains is to install a standpipe overflowwhich can be detached to allow complete drainage of the pool.
If the pool is small and there is no outlet (a storm sewer or lower land) for a gravity water, it is still worth installing a sump to which water drains from the floor of the pool. Bottom drains are often connected to filtration systems. but are also useful for occasional cleaning in biological ponds. In the long term, ponds (and lakes) are temporary phenomena. They fill up with debris.
Bottom drains work best when protected with with a domed cover. This collects debris for removal but prevents it from blocking the pipework. Domes are also available for vacuum pumps if the pool does not have a bottom drain.
From top to bottom: (1) suction strainer on pool floor (2) overflow drain setting water level, removable to drain pool(3)bottom drain on pool floor (4) sump set into pool floor