Chameleon plant, water bacopa, and verigated water celery are hardy low growers, but the last two can be invasive if not kept in check. They spread fast with runners that will root in anything. If you can go a little taller, up to 18", dwarf cattail or corkscrew rush are good choices.

Dwarf sweet flag. I have the varigated stuff. Love the lime green color. Planted bareroot in gravel along edge. Is supposed to be a good filtration plant as well. Hardy to Zone 3 so can overwinter in the pond.

I love marsh marigold. I'm not sure it fits ALL of your requirements, but I have the top of my pot a few inches below the water level and the leaves sit about a foot above, maybe a little more. It over winters fine, is the first flower in my garden, SWALLOWS excess nutrients whole, has eliminated any spring algae bloom and provides great cover for the frogs.