I totally agree with using a kayak, I have a Wilderness Systems Pungo 120 that I use a LOT. It is wide and stable. A couple things that help - one place I go is a state park that doesn't allow motorized boats, so I don't have to contend with wakes. Also, I keep my camera (Z9 + 180-600mm) in a dry bag if I'm only paddling. Only once I get to where I want to photograph does it come out. Also, I got a paddle leash so that when I set the paddle down to photograph, I don't have to worry about the underbrush snagging it and leaving me stranded.
Back to the original topic though, I think the 180-600 is a great budget starting point. As someone mentioned, pair it with the Z 24-120, and you'll have a great range that can cover environmental wildlife (as well as landscape, if you choose), and should keep you just under your budget.