According to the Great Bahuna of the Order of TransAmerica Cyclists:

One must dip the rear wheel and rear wheel only in the starting water body.
One must dip the front wheel and front wheel only in the ending water body.
Dipping the entire bicycle is considered in poor taste - -
However, sneaker waves have accomplished the same end at times.

You do know that the official TransAm Trail ends at Yorktown on the York River -
Not on the Atlantic Ocean. You can get there, but it's a tough ride thru Norfolk.
Anyhoo, the York River is a wide salt-water estuary with sand beaches.

My suggestion -
Strip off the panniers and carry the bike to the water in both places.
Watch for the wave pattern and stand at the waves' edge.
Have your photographer snap the shot right as the wave is touching the wheel.
(This is a little tough to carry off with a timer.)

Here are maps of Fort Stevens in Astoria and Yorktown:





The wreck of the Peter Iredale is a nice spot on the Pacific.
If you camp at Fort Stevens you can just ride out from the campground.
If you stay elsewhere, you can ride out, leave your panniers
in the parking lot, and carry your bike to water's edge.

In Yorktown, you can head to the base of De Grasse Street.
There's a nice, convenient zig in the pavement at river's edge -
Not large enough for a parking place, but for your panniers, yes.
Then it's only 50 feet to the water.
Estuary sand is even finer than ocean sand -
So carrying your bike is extra smart.

Have a great trip!