Well, I just learned something new! Bigleaf Hydrangea is extremely salt tolerant! Sounds like the sun exposure is fine, partial shade is best anyway. I would build a plant bed by 'fruffing' up the soil, double digging and allowing it to become a raised bed! Are there any trees near the area you want to plant? Roots of Hydrangea are only a few inches deep so if you make a raised bed (no need to put sides on it, just a little trench at the bottom 8" wide and 6" deep) to carry away excess water. Careful with any mulch, you do not want to bury the roots any deeper! Way cool!! How about adding a Salix tortuosa Mansudana...a curly branched willow nearby? They grow very quickly and can add a bit of shade later. Just an idea.
So this is freshwater seawall. Just start double digging the bed and allow the soil to get 4'! It settles down quickly and the plant settles with it...it'll probably be only a foot high. Check the pH of the soil, hopefully it will be more acidic; 6.2 - 5.5? I am sure there must be a lawn so be careful that the lawn fertilizer doesn't get on the soil for the Hydrangeas, or few flowers. Test the pH of the lawn as well, that soil should be more alkaline; 6.5-7.1? If the soil of the lawn is acidic you need to lime the lawn. If the soil for your Hydrangea bed is alkaline, you can slowly decrease the pH using sulfur.
The best planting of Hydrangeas would be a mass planting to form a 'sweep'. Unless you are planting just one. Use unequal triangles for placement, nothing in a straight line. Allow room for the size of hydrangea you use. For instance a plant 5X5 should be planted 3-4 feet apart. Hydrangeas are easy to control size but shallow rooted plants only use the top few inches and would compete if too close. You want a mass of hydrangeas that run into each other. USE only ONE SPECIES. At their feet you could plant Sweet Box, Lonicera pileata something that grows only 2 feet and the flowers are insignificant. A 'green frame'? Sweet box (Sarcococca sp.) is delightfully fragrant. Loves shade so if the hydrangeas are on the south they will shade this plant. Lonicera pileata has a wonderful horizontal branching and is stunning grown in a mass planting. This plant will thrive with more sun. Create an 'eddy' to situate a few adirondack or rustic chairs, little table, to get closer to the water for afternoon cocktails??
