I moved into a new house with a large pond that is completely taken over by a mystery plant. I can't find it in any aquatic plant database on the internet and I want to know what it is before I jump into yanking it out.

My guess: Water Celery.

You're location, or at least a zone could be of great help. Water celery does look like it could be a match.

I'm in Seattle, WA.

Oenanthe sarmentosa
pacific water parsley

It is in the Washington native plant society web-sight.
Sounds like a common plant in your area

If that is the same plant as Water Parsley there are zero photos in the ATP database.
Water Parsley (Oenanthe sarmentosa)

Thank you!!! That's definitely it. I was seriously stumped. I grew up in the Midwest and had never seen anything like it! I'm very glad to know it's native.

just wanted to say welcome to ATP
Lots of friendly people here always looking to talk plants

Warm welcome and please wander over to the PNW forum when you get a chance. Since you are new to this area, that's a good place to get accustomed to what will (or will not) grow around here and/or ask questions about your new growing region.