Its not just the water . Hydrangeas are woodland plants, they need hummus rich damp soil and dappled shade . You have them in full sun with poor, well drained soil. They are beautiful plants, just move them and make them happy.
And I’d fill that border with a mixture of evergreen and deciduous super tough shrubs with different shapes, habit, colours and textures that will cope with being cut back to fit the space , won’t mind the wind and will need very little maintenance
eg
cotoneaster, lonicera Baggesons Gold, Viburnum Eve Price, Buddleja Nana, phormium, brachyglottis Sunshine, Potentilla ( shrubby not deciduous ) , Choisya ternata if it’s not TOO windy ( not the yellow one called Sundance as it won’t like the exposure ),
All with work well some of the grasses and prostrate rosemary that Pp have suggested. They are all cheap and readily available. As the roots grown they will help bind and stabilise the soil.
Dont get one of each type of plant , repetition looks better along a border.