It was important to identify the location of Zarahemla before trying to identify the East Sea. The reason why will become clear in a moment.
By looking on a map for a body of water east of Aurora we find Leechworth State Park:
It is considered the Grand Canyon of the East with the Genesee River running through it:
Further down the river in Rochester is the Veteran's Memorial Bridge:
This is the view of the East Sea/Genesee Gorge-River from the bridge. In Book of Mormon times, the banks were bare and treeless. If you could see through the trees, you would see steep cliffs. This Gorge, like the Niagara Gorge was not something people would cross on foot, baptize in, or fight upon the banks thereof. It just wasn't possible.
And finally, this is how the river looks when it empties into Lake Ontario:
Depending on where one views the Genesee River, they will either be awed and call it something more than a river (Like at Leechworth State Park, Veteran's Memorial Bridge.), or just a river.
The fact this was their eastern border and not the River Sidon is substantiated by the lack of evidence:
The evidences of mound culture are more numerous in extreme western New York than east of the Genesee river. (William A. Ritchie, Pre-Iroquoian Occupations of New York, 1944, p. 9)
Back then, the East Sea extended further west as we have identified on the Narrow Passage page.