Umbrella grass is one of the common names for Cyperus. It's not really a grass, but has some common characteristics of grass.

Cyperus is typically a tropical perennial, seldom hardy below zone 8, but there is at least one variety, Cyperus albostriatus Variegatus, which is rumored to be hardy to 7b. I'm testing it this year, so I'll see how it does. I'm going to leave some in the ground, and bring a couple indoors for winter, as it is also a popular houseplant.

You can find plenty of photos using Google Images.

When I Googled the umbreaal grass I found Egyptian Papyrus. I have some if you would like some for postage let me know. It goes by many names. It is called umbrella papyrus. It will grow in or near water or anywhere else. The cold kills it back here in zone 8b but it comes back every spring. I can send you some cuttings with rooting directions. It is the easiest thing I have ever rooted. Email me.

Is this the plant you saw?

Here is a link that might be useful: cyperus alternifolius

grass guy, it looks alot like what I saw but it wasn't in water. It was planted in a flower bed.

Doesn't have to be in a pond, but does prefer wet toes to keep looking nice.

I have some green Cyperus that looks just like the link above, here in zone 7b/8. I have had it for about four or five years. It's tough. Mine is in a mixed woodland border and gets moisture from the path the air conditioning trickle takes, but is at least 6 to 8 feet from the source. I have not had a hard winter in several years and I do mulch heavily.

I have had it growing in my yard now for 6 years and there is no pond or anything else like that. It is also called Egyptian Papyrus. Why two different names I do not know.

Cyperus and Papyrus are two different plants. They are similar in that they will both grow in standing water and they are both tropical but they aren't the same plants.

Here is a picture showing both of them, side by side in my greenhouse pond. Egyptian Papyrus on the left, Cyperus on the right.