Bottom drains, as Gregg says, are really not that difficult to install .....ON A NEW PROJECT. Retrofitting a BD on an existing pond can be a real PITA.....pond needs to be drained; liner and underlayment must be pulled back to proposed location of BD, trenching must be done for drain line, etc. De-construction and re-construction of a goodly portion of the pond is unavoidable.
Although the piping will be exposed, Tetra makes a BD that can be placed on top of the existing liner.
"Everybody is ignorant, only on different subjects." - Will Rogers