According to Aquarium Plants (Barry James) (a nice little book, if you can find
it) Marsilea crenata is a tiny creeping plant up to 3" high that remains
permanently submerged. It comes from SE asia.

I have experience growing Marsilea vestita as a groundcover outdoors. It is
native to western N. and S. America, deciduous and rapidly spreading in moist,
sunny areas. Also, fairly invasive. It, too, has neat leaves, and might
survive submersed.

Regards,

Richard Fee