Well I hate to blow your theory out of the water, but if your calculations are correct than it would take longer for you to boat from Center Harbor to long island than it would from long island to Roberts cove because you would be going “uphill” from center harbor to long island and “downhill” from long island to Roberts cove. And from experience this is not true!
Also if you had a water level 16.6 miles long, and held one end at center harbor and the other at Roberts cove the water level would be perfectly equal, or level. So in conclusion the earth is flat! Not round as previously stated.