Species-wise I'm leaning towards juvenile Hooded, but not ruling out juvenile Red-breasted. Anyhow, it was rather approachable - much more than I'm used to for any merganser. It was also rather predictable in habit. Spending lots of time diving in very shallow water under the duckweed (12-18 inches deep) it would "stand" and shake it off every so often. This is one of those instances, just after it shook its' head. BTW I purposefully kept the single speck on its' forehead as a sign of it being completely submerged before.
Being very overcast and sombre conditions I wanted to use flash, but lying flat on the boardwalk with the lens poking between the floor and lowest beam the flash was smack-dab beam height thus negating any of its usefulness. Plan B, overexposing the duckeed to get correct exposure on the merganser. The resulting RAW had a near-perfect subject but almost completely white surroundings which were then brought back to correct levels during post-processing.
Canon 40D + 100-400L @400mm, manual exposure, 1/200s., f/7.1, ISO 500, +1 EC metered off duckweed, handheld, just about full-frame.