It was called Lake Missoula, a freshwater inland sea, more than 600 meters deep at its deepest point, holding more water than Lake Ontario and Lake Erie combined.
The perfect ships for moving across the calm waters of an inland sea like the Mediterranean but pretty terrible for moving across the unpredictable and often harsh waters of the open ocean.
These are the largest collection of freshwater lakes anywhere in the world by surface area that pretty much act in a similar way as an inland sea does like the Mediterranean or the Baltic.
This warming may have been caused by the drying of inland seas and release of methane, a potent greenhouse gas, from ocean sediments, Even among these other warm periods, you're making a false analogy.