I predict that when all fossils are collected (whenever that time may be), the mother of all birds (an archosaur ancestor to birds) which needless to say, dating back at least to the Triassic and predating the theropod dinosaurs, will be discovered to have inhabited semi-aquatic environment.
Archeopteryx
Archeopteryx
Habitats of the Archosaurian early ancestors to birds.
Further about the aquatic environment of Archeopteryx and where the remains have been found... in Lagoons.
The early world was filled with such habitats, some living between terrestrial, semi-aquatic and aquatic lifestyles.
The argument for a "wading" Archeopteryx may be weak, but the conclusions for "Aquatic Ancestors" of Archeopteryx are strong:
From Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds, By Gregory S. Paul
"Swimming" and "Flying" require the same physics (Hydrodynamics vs. Aerodynamics).
Fruit flies 'swim' through air, using the same physics as fish, study shows
Fruit flies and other flyers also use drag to "swim" through the air, the scientists say. Their discovery lends support to the evolutionary theory that flight in insects emerged from swimming.
The same applies to snakes, which are reptiles that have been skilled swimmers for millions of years yet have neither feathers nor wings and have learned instinctually to seize upon principles of aerodynamics to "swim through the air".
Swimming... gave rise to the instinctual desire to fly. Most birds are either swimmers, or flyers and have been interchanging between them for the past 300 million years. The evidence found in Archeopteryx reveals that he, like his ancestors, never wandered far from the water's edge.
Embryonic Evidence in Birds and Reptiles
"...He examines in detail the expression of evolutionary stages in the development of embryos, tracing from the process of cell division to the development of specific anatomical features. He finds a striking resemblance between the embryonic development of reptiles and birds, including details of the skeletal anatomy (with special attention to the hands and feet) and various organs. He notes that bird and reptile embryos develop visceral arches, hinting at their aquatic ancestry. Of more interest to his goal, Heilmann writes in a similar vein that the embryos of certain birds clearly show a three-clawed finger structure, at least one of which (the hoatzin) retains actual claws after hatching. He mentions other anatomical features of bird embryos that hint at their reptilian ancestry as well, such as the embryonic splitting of the pygostyle into distinct separate vertebrae."
(Source)
Visceral arches:
"...Columns of mesenchyme found in the neck of the developing vertebrate embryo derived from cranial neural crest. In lower vertebrates, blood vessels formed here become part of the gills; in higher vertebrates derivatives include portions of the jaw and middle ear. Also known as branchial arches, gill arches, or visceral arches."
(Source)
"Mesenchymal structures in the region of the embryonic pharynx and visible on the ventrolateral aspect of the head; give rise to skeletal elements, larynx and other structures of the head."
(Source)
"...the “pharyngeal apparatus” consists of a series of paired pharyngeal arches and fissures which develop on the exterior with a corresponding set of pharyngeal pouches on the inside of the throat, separated from the external fissures by a thin membrane (more on the details in a moment). And in fact the possession of these structures at some point in development, along with a hollow dorsal nerve cord, a notochord and a post anal tail, are the defining characteristics of the phylum chordata to which we and all other vertebrates belong."
(Source)
Just look at those wonderful visceral arches and how even humans retain the innate ability... to swim at birth! This instinct is written in the genes of all animals.
...Even humans, seeming so far removed from their ancient mammalian ancestry... are born with a rudimentary reflex to swim... inherited from a distant water-borne ancestor somewhere in the murky evolutionary past.
Why would a human need "Visceral Arches" --GILLS??!
And if humans retain this ancient swimming reflex, -- then why wouldn't birds?
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