It could be arrowhead. When my arrowhead sprouts up, they have a more spear shaped leaves, but then start to develop into the flying "V" before they get leaves longer than a couple inches.
It could a be pickerel weed or, maybe, lizard's tail. Either way, keep an eye on it and post some more mature pics, especially if it flowers.
I don't think it's water hyacinth or at least not the floating kind.
If it's pickerel weed (my bet), you are blessed to have it. Especially if you like Arrowhead. They both live in the same locations of the pond and flower nicely.
Here is a pic of the only bloom to date from some of the mystery plants that you gifted me. I thought it was going to be Water Arum, but the flower has set me back. I am currently at a loss...anybody?
I got it - 99% sure...Heteranthera Reniformis or Kidneyleaf Mud Plantain or Floating Leaf Mud Plantain. Seems to be native to North, Central, and South America, but listed as endangered in Illinois and Ohio and of special concern in Connecticut...
One thing I did read about it said that it has a bad time competing with grasses, rushes, and such as they tend to block the sun from getting to these low lying plants which chokes them out.