Water under liner while building

[GBBUDD]

you don't need drainage beneath the pond if it's full of water because the hydrostatic pressure of the water in the pond is equal to the same water outside the pond

Id agree with 75% of this statement but as some who has been drilling for the last twenty years and have struck aquafirs numerous times. The hydrostatic pressure is generally the same in high water table areas. However we have hit springs where we interrupted the flow with the excavation and the force of the water from below can easily outmatch water sitting on a liner even with ten feet depths.
Cement ponds gave been known to actually float within there excavation , though admittedly rare. But not so rare that they build in a pressure relief plug at the bottom. So a a Edpm can easily get out matched by the waters below.

Its far easier if there's a question to simply add the path of least resistance. That being sand gravel or pipe for water to find a way out.

 

[hammer]

nothing wrong with adding the drain, I imagine though, as long as the surface water of the pond is above the ground water elevation, draining the soil beneath the pond isn't necessary unless you intend to drain the pond. It seems what they have, is the bottom of the excavation clipping the top horizon of the ground water.

Another option is to monitor ground water before deciding if the drain is needed or if no drain is installed to ensure ground water is low enough to drain. During pond excavation, install 2" or 3" (vertical) pvc pipe next to the pond, make sure it's deep enough and the bottom has 12" of stone beneath it.

Venting ground water to a 3" vertical pipe at 3' generating an artesian well with the pressure to flood the yard at surface level seems like a wetland... or the spring/aquifer is capped with rock or highly compacted silt/clay.