There are a couple lists of wet-tolerant plants in our newly-revised Groundcovers web page (separated by sun vs. shade and native vs. non-native), though few would be available as seed. Instead, they may be found as either plugs (small-sized starter plants), especially if grown by a native plant nursery, or larger plants at other nurseries/garden centers. Plugs would be the easiest to plant, and they establish fairly quickly for their size. You probably would not be able to sow seed directly onto the water surface, but could germinate them in containers first to get plants large enough to transplant, or sow them onto damp soil but not open water.

Moss is sometimes sold in spore form as "moss milkshake" type products to sprinkle over bare ground to "sow" them, but it's hard to know what species among the hundreds that occur in Maryland would be best suited for the site, since soil characteristics also play a role in which species thrive in certain settings. (For example, they don't all need acidic soil conditions.) For moss, it's simplest to just let locally-native species self-sow via spores into areas suitable for them.

Miri