Tidewater can be defined by different boundaries, depending upon your point of view.
A geologist may define Tidewater as the area east of the Fall Line, east of the crystalline rocks of the Piedmont. An astronomer may consider Tidewater to be the area where the water level rises and falls each day based on the gravitational pull of the moon (and the sun, too).
An ecologist may start talking about the Chesapeake Bay estuary, or the downstream end of watersheds (including the mouths of the rivers) east of the Fall Line. A historian may consider Tidewater to be the location of early Colonial settlement around the Chesapeake Bay up to the 1730's. (In that case, Alexandria would be not be a Tidewater community, because Alexandria was not chartered until 1749.)
A voice coach might recognize certain accents ("let's go to the riv-ah and catch some fish...") as being associated with Tidewater Virginia. A politician, business owner, or government official in southeastern Virginia may tell you that Tidewater is a region with a complex mix of official jurisdictions; the waterways and history of settlement have fragmented the communities into separate cities/counties.
If you drive down I-64 east of Williamsburg, it's very difficult to identify the boundaries of Newport News, Hampton, Norfolk, and Virginia Beach. You may see the signs to Poquoson... but how do you know you've reached it?
If you head west from Virginia Beach, the edges of the political jurisdictions of Norfolk, Portsmouth, and Chesapeake are no more obvious than what you saw driving east. The region's political boundaries resemble Los Angeles, with separate urbanized jurisdictions adjacent to each other and little undeveloped farmland separating the cities. The city lines blur in a sea of monotonous urban development - except for the Eastern Shore. That area is still undeveloped at the moment, though a reduced toll on the Chesapeake Bay-Bridge Tunnel may encourage more commuters to live in Northampton County.
When Tidewater Virginia is defined as a cultural region, the boundaries lack scientific precision. Some people consider the Eastern Shore, the Northern Neck, the Middle Peninsula, the Peninsula, and the portion of Hampton Roads south of the James River to be "Tidewater." Others think "Hampton Roads" and "Tidewater" are the same, excluding the Eastern Shore.
Some consider Tidewater to be just the portion of Virginia that is:
1) east of the Fall Line, and
2) between the James and Potomac rivers, and
3) west of the Chesapeake Bay
That definition would exclude the Eastern Shore, as well as Virginia Beach, Norfolk, Portsmouth, and other jurisdictions south of the James River. You may hear people discuss "Tidewater and Hampton Roads," to include specifically the Chesapeake Bay watershed south of the James River.
All definitions are correct - and all definitions are wrong. Cultural regions are defined in different ways by different people, and there is no official judge sitting in big chair at the front of an International Court of Regional Boundaries to make "official" decisions.
What's missing? The cities of Manassas and Manassas Park, the counties of Loudoun and Fauquier - and other jurisdictions in the Piedmont, Blue Ridge, and Valley and Ridge provinces, where the rivers drain into the Chesapeake Bay but the water levels are not affected by tides.
necks in York County (Back Creek)
Source: National Atlas
Even defining the edge of Hampton Roads can stimulate a geography debate. The Hampton Roads community is split into two major components, as represented by the location of the regional airports - one on the Peninsula at Newport News-Williamsburg, and one in Norfolk. If someone south of the James River says they are going to the airport, you can generally assume that they are going to the Norfolk International Airport. Someone living north of the James River will probably use the Newport News-Williamsburg or Richmond airports, avoiding the traffic delays at the Hampton Roads Bridge-Tunnel.
The airports in Newport News-Williamsburg and Richmond are growing as the population in Hampton Roads increases. However, so long as the region has three small airports, Hampton Roads is unlikely to attract a carrier to offer nonstop flights to Europe. Southeast Virginia residents will have to keep going to Dulles or Raleigh-Durham airports to fly direct to London; the "international" in the names of southeast Virginia airports reflects the ability to reach other countries after stopping at another airport.
Another clue to the impact of the waterways fragmenting the population - there is no culturally-unifying professional sports team in the region. Back in 2000, one source described the problem as follows:2
Regional transportation projects can knit together the interests of different jurisdictions in a region, but getting separate jurisdictions to adopt common priorities and then direct funding according to those priorities requires a structural, long-term approach. In 2014, the General Assembly created the Hampton Roads Transportation Accountability Commission to do just that.
The mayor of Virginia Beach proposed a quicker approach. He offered a marketing proposal to re-brand the region by changing the name from "Hampton Roads" to "Coastal Virginia." An official at Old Dominion University then suggested renaming the region with a more-specific title, "Virginia Beach-Norfolk Metro Area," because:3
A local banker then carried the concept to extremes, perhaps only partly in jest. He suggested lumping Williamsburg and Richmond with the rest of the Peninsula and South Hampton Roads:4
It was not the first effort to rebrand the region. Business and civic leaders lobbied successfully to have the US Post Office change the postmark from "Tidewater" to "Hampton Roads" in 1983. In 1996, the Hampton Roads Partnership initiated a rebranding campaign. Rather than refer to Tidewater, Norfolk-Virginia Beach-Hampton-Newport News, or just Hampton Roads, the Hampton Roads Partnership proposed adding the tag line America's First Region.5
In 2014 the Hampton Roads Planning District Commission conducted a poll on how residents within the region described where they lived, when traveling outside the region. The intent was to determine if there was a consistent "sense of place," and if a regional identity was more important that the particular jurisdiction where a person lived.
As a new descriptor, "Coastal Virginia" had few advocates. The name of the jurisdiction with the largest number of people, "Virginia Beach," clearly dominated over "Norfolk area" - and only 5% described themselves as being from "Tidewater."6
1996 rebranding focus: add America's First Region to Hampton Roads
Source: Hampton Roads Partnership, The Campaign - Hampton Roads, America’s First Region
Kingsmill and other retirement communities around Williamsburg are adding suburban growth to the already-urbanized areas at the southeastern tip the Peninsula (the land between the York and James rivers)
Source: ESRI, ArcGIS Online