Anhingas ("snake-birds") are almost unique among waterbirds: their plumage is wettable rather than oiled fully waterproof, so water soaks into the outer feathers and displaces the air layer that gives ducks their high-riding buoyancy. That is why they swim body-submerged with only the neck showing, and why — unlike a duck — they must dry their wings before they can fly.
Net vertical force on the body follows Archimedes' principle with three terms, evaluated every frame from the "feather wettability" slider (fraction of plumage volume that has taken on water):
F_buoy = ρ_water · g · V_submerged (upward, Archimedes)
W_body = m_bird · g (downward, body weight)
W_water = ρ_water · g · V_feather · wetness (downward, absorbed water)
F_net = F_buoy − W_body − W_water
The bird settles (or rises) until F_net ≈ 0, which is solved implicitly each frame by integrating vertical acceleration — at high wettability the extra water weight cancels most of the buoyant lift and only the neck stays above the waterline; at low wettability it floats much higher, like a cormorant fresh out of a low-wettability moult.
Underwater pursuit uses a drag-vs-thrust swimming model: propulsive thrust from foot-kicks accelerates the bird toward the fish, opposed by quadratic drag F_drag = ½·ρ·C_d·A·v² (webbed feet, streamlined body), so top speed is where thrust and drag balance rather than being an arbitrary cap.
After surfacing, wing dryness increases over real time (faster in the drying-pose with wings spread) and Attempt takeoff only succeeds once dryness and buoyant load both clear physiological thresholds — a waterlogged anhinga genuinely cannot get airborne.