Apart from Egeria densa the only other true aquatic C4 plant currently known is Hydrilla verticillata.
However some aquarium plants are not considered aquatic by botanists, such as some Eleocharis and Cyperus species which are C4.
At least one species, Eleocharis vivipara uses C3 in its submerged leaves and C4 in its emergent leaves.
There is another category similar to C4, called CAM (Crassulacean Acid Metabolism).
CAM plants uptake CO2 at night and stores it as malic acid. Then extracts and uses the CO2 during the day.
CAM plants are typically in arid terrestrial environments, but there are an aquatic versions in the genus Isoetes, commonly known as "quilweeds."
Aren't Vallisneria sp CAM plants as well? I remember reading it somewhere earlier and found an old article substantiating it (CAM Photosynthesis in Submerged Aquatic Plants), as well as saying that Sagittaria and Littorella used CAM too, but maybe there are more recent results?