I accidentally dropped some weed stems and grass planks in the water at the pond edge. They are just under the surface. The pieces highlight and the button to pick them up ('E' - on PC) shows up but is grayed-out. They are far enough down that my character is in "swim mode" to get the prompts to appear. This may be by design but is frustrating to have them just sitting there unusable and for the game to imply that they can be picked up when they actually cannot be picked up. 

I've had the same problem, not in the pond, but at the shallow edge of the water in the area near the bird bath.  Dropped 4 stems and cannot get them, and they haven't floated up to the surface/come over to the edge like they used to. 

I'm sorry you've both been experiencing this issue. The team is aware of it and will be working on a fix for an upcoming patch; if you haven't already, please take the time to submit a support ticket so we can track this issue and follow up when it is resolved. 

Thanks