The northern portions of the Suwannee generally have reduced visibility due to the tannic acid you mention (the origin of the Suwannee is the Okefenokee Swamp!!). However, down river from about Branford the visibility tends to increase due to the dilution from the many springs that feed the lower half of the river, as well as the Santa Fe, which in turn is fed by the Itchetucknee (lots of clear water).
Of course this generality goes out the window when the thunderstorms pass through, though this usually is more of a local affect and short lived. The best time of year for diving the Suwannee is the winter--before the normal late winter rains start (which has not been the norm the past few years). In the winter outflow from the swamp is minimal while the springs continue to do their thing mostly unabated--which means the lower river has excellent visibility.
Due to the larger volume of spring water, the temperature is also moderated and we use the same wet suit protection, as the water is little if any colder than in the summer. Another plus--in the winter boat traffic is almost nonexistant.
As for artifacts and fossils--countless examples have been recovered over the years--having had a diving love affair with this river since the 60's--I probably have found my share!!
