Surely you must be
When in red, I was very loud.
In high definition, a treasure.
You put me in to interfere.
to "put an oar in" is an idiom for interfering in other people's business
Rest on me for your leisure.
"rest on your oars" is an idiom as well.
Put me between the last two letters,
I am an Asian plant.
Solved by @Stiv in the comment section (would never have gotten it myself):
The last two letters are, in this case, "zed" and "y", giving zedoary.
Keep some of me in the water,
And being unsettled you can't.
If you remain calm and collected, you "keep both oars in the water"
Title: "Sometimes with Feathers?"
turning an oar in a particular way when rowing is called feathering the oar.