It kinda looks like Temple Plant - Hygrophilia corymbosa or other variety. Search through some pics of it emersed and submerged growth.
If not...
Were these growing in an aquarium under the water or out of the aqaurium, a fish pond or out in the wilds? They look like marginal plants.
If you found them in the wild can you describe what state and the environment where they were found? River, pond, other details as in - in the water, waters edge, but roots in the water, etc.
These kinds of details will help narrow it down.