What is the easiest way to create a polygon that defines a coastal research area?

[ParisJennings]

I need to create a polygon that outlines the study area for research on bottlenose dolphin populations. It would need outline the coastline and extend to a defined buffer distance offshore. I have coordinates for the northern and southern boundaries. What would be the easiest way to go about this, without drawing it vertex by vertex?

I'm going to use ArcMap 10.6 or ArcGIS Online.

[deleted-user-qpvAI3Fo0MKR]

I'd start here - NOAA Shoreline Website, I see that they do indeed have shoreline vector data, it's split into the following groups:

Clicking on any of the links brings you to the corresponding USGS page with the data download link:

Hope this helps.