Where can I go to see if the Coast Guard can install some navigation aids in Lake Maurepas in LA?

Question

There are unmarked stump area at the mouth of a river there.

Answer 1

The Coast Guard operates in ocean waters and navigable rivers (meaning there is commercial traffic). They don't do lakes.  I would contact whatever agency you would call if you were sinking.  They have the jurisdiction.

Or, you could flag it yourself.

Answer 2

Lake Maurepas actually is considered "navigable waters" by the USCG, because it connects to the Gulf via Lake Ponchartrain. Here in Seattle we have two lakes which are only accessible via a canal and locks, and they're also "navigable waters" (and, oddly enough, "International" waters vs "Inland", so far as COLREGS is concerned). The phone # for USCG 8th District (that's you) aids to navigation  office is 504-589-6277.