While mulling the matter over, I was casting around for a candidate that would best fit the odds-and-ends of the Great Lakes whale reports; something that could look like a dolphin sometimes, like a grey whale sometimes and even resemble a sperm whale to some witnesses.
I began to suspect that a North Atlantic bottlenose whale might meet the specifications. Bottlenose whale remains are found in the inland sea deposits when the great lakes were proglacial, and
the bottlenose is a good medium lengeth at about 25 to 35 feet long, which could be mistakenly doubled to 50 to 75 feet long by the witnesses and fit the larger reports well enough. The whales could be considered the replacement for Mackal's [Eastern] Basilosaurian lake-monsters but they would not be Naiatakas: they would be Bessies. Lake Erie's 'South Bay Bessie' is often reported as a large spindle-shaped creature in the right size range, and sonar targets also fit the description. That does not mean to include ALL of the Lake Eerie reports, but the 'Whale' sightings are a remarkably good fit (and it does not matter if the reports are often string-of-buoy reports; those would be the waves in the wake. This time at least the creature would be big and powerful enough to leave a very noticeable wake).
Some of the fossil whales of Michigan have very recent C14 dates, up to possibly the Viking age or even colonial times. These unusually recent dates are thought to be due to contamination of the samples (link cited for Michigan fossil whales last time) However, it can also easily be turned around to say that there is evidence for recent survival of whales in the Great Lakes.
Sightings have been in decline for some time though and if the Great Lakes whales really are represented by 'Bessie' sightings, the population may very well be in peril.