It won't be being near the coast, these plants are fine with that. They do, though, prefer a bit of shade, so if yours are in full sun, they won't do so well. They like moist soil that doesn't completely dry out, so it may be that they get too dry. If the leaves look yellowish or mottled in colour rather than green, a shortage of iron could be the problem.

I,d make sure that it gets plenty of water, mine always seems to need a lot especially in hot weather when the leaves quickly droop, also do you feed it at all?, I give mine a balanced fertiliser in the spring

Are you pruning it in winter? You may be cutting off some of the flowering shoots. Still needs water and perhaps fertiliser though.

Depends on the hydrangea you have, but on the whole they do like compost/manure dug into their roots, especially when planted out, and a moist soil as recommended above. Blue hydrangea only stay blue in an acid/ericaceous soil. There are compounds you can buy to keep blue ones blue. Sickly plants are generally missing some vital elements. Try a manure compost and a good rose feed/tonic, or ericaceous (iron based) feed for a blue hydrangea. Prune down to about 3 buds now, or just take off the dead from last year, depending how big a bush you want and flowers. Bigger bush usually but not always means more but smaller flowers. Smaller bush usually but not always means fewer but larger flowers.