This ground cover is about 4" tall. It has blue flowers (see second photo).

It's a Veronica but I can't give you the precise species.

creeping jenny - fast spreader, roots easily, g good as groundcover, but doesn't play well with other plants as it spreads up & over works fine with large shrubs-- just pull them back down if they climb a branch snip with scissors if you want to keep it contained prefers moist part sun/shade nice trailing out of mixed containers

If you live in New York as suggested in your name, you might want to search on cliffandjoann. They built a stunningly beautiful pond and have explained the details. They say that some of their plants are in pots. I use impatiens and am under the impression that they will grow without much actual soil. It is certainly true for water cress (from the grocery store) anchored directly in flowing water. For something larger, sweet flag will certainly work. You can also use vines or other plants growing beside or near the area. You can also add logs or other natural materials for interest and coverage.