Agree with many already suggested .....eryngiums are a must but check out eryngium Jade Frost. ,it's a beauty and different.
Mesembryanthemums, gazanias, etc. look so good in gravel. They love the reflected heat from,the sun. There are some stunning upright osteospermums too....perhaps a little tender for some areas but so easy from cuttimgs that they can be kept from year to year. Linum perenne, the blue flax, has womderful glaucous delicate foliage and the most exquisite blue flowers all summer. Can you plunge aeonium Schwarzkopf there too?
These may not be "beach" plants but they look good in gravel. The Noah's Ark Juniper.....sorry name escapes me for mo.....is a small upright, slow growing conifer that I Iove on the rockery will look nice in gravel. For different shape another juniper...Blue Star....is a blue foliaged bun shaped plant. a yellow leaved santolina called lemon fizz is worth checking out too. I grow this on a free draining wall and it's evergreen bright yellow foliage is charming. How about Tulbaghia? Silver/blue/white/grey slender foliage with pink flowers ....again set in gravel looks very nice. Many hardy geraniums ESP the cushion ones will look good.....black Beauty is a gem with purple compact foliage. and gentian blue flowers. Arabis Ferdinandi coborgii (?) is a lovely variegated Arabis with sparkling white flowers in spring.
Getting carried away here and strayed off "by the sea" request.
(juniperus communis compressus....Noah's ark juniper. Had to look it up)