How tall an ornamental grass are you wanting? Pennisetum alopecuroides/fountain grass grows to about 2-2 1/2 ft. in height. Kalamagrostis/feather reed grass grows to about 5 ft. in height but doesn't have (in my garden anyway) the lovely fountaining habit--it remains pretty much vertical right into the fall. I'm assuming you have sandy soil but if it's not, that might widen your options.

You might Google ornamental grasses and check out those that prefer moist, sandy soil or else visit Santa Rosa Gardens' website and review the recommended growing conditions for their fairly-extensive selection of ornamental grasses.

I will comment that my own experience has been successful but limited to only those named above. The only variety I've grown that prefers shade is Japanese sedge grass. That said, there are several available cultivars + I have mine growing in full shade, part shade, part sun and all are thriving.

As for a perennial that might suit your needs, I'd look at those that might thrive in your situation. Here are a few suggestions to get you started:

Armeria/sea thrift
Agastache/hummingbird mint
Alchemilla mollis/lady's mantle
Athyrium/fern
Anemone hupensis/Japanese anemone
Chelone lyonii/turtlehead
Cimicifuga racemosa/black snakeroot
Dicentra spectabilis/bleeding heart
Hellebore/Lenten rose
Hydrangea
Pulmonaria/lungwort, Bethlehem sage
Tricyrtis hirta/toad lily
Trollius ledebourii/Chinese globeflower

There are likely lots of others but these have been reliably hardy in my Z6 garden.

Best of luck to you!

Here is a link that might be useful: Santa Rosa Gardens

Beach rose, or rosa rugosa, tends to run - I don't think I'd plant it near a shower because it will come up just where you want to walk barefoot.

Tall grasses would be nice along the fence - but it's true that most of them like a lot of sun. it's hard to know what 'part-sun' means - can you tell us how many hours of sun the area gets.

If I had a soggy area, I'd plant one of the giant-leaved, dramatic plants like Ligularia dentata, Astilboides tabularis, or one of the Rodgersias. You might not like that sort of bold foliage - it's a matter of taste.

Lots of hydrangeas prefer part shade, would those work for you? They're really easy, and they do like a lot of water - though not constantly wet feet. Not sure how boggy this area is.

Fine Gardening magazine's plant finder works well for this kind of search, I'll link to it.

Here is a link that might be useful: Fine Gardening plant finder