While I have no formal interior design training, my experience leads me to offer the following considerations. First of all, I think you have done well to try and maintain the view from that window and I understand your motivation to not obstruct it. That said, I think it's a large enough window that it can afford to lost a little of it's visibility for the sake of allowing the space inside to function better.

I personally am not a huge fan of seats without backs... they aren't terribly comfortable and consequently are not as useful. I think you could easily put a very low-backed bench or settee in its place and only obstruct the lowest portion of the views, which are of the deck, rather than the lake.

The other thing to consider is that there is a floor register under the window. While I think a bench/settee will work, you may want to consider using a round table flanked by two chairs (something light-weight... not too massive). This would still allow for a limited amount of seating, would be less of an obstruction to the heat/AC coming out of the floor register, and a slightly taller piece of furniture (table & chairs with a simple bouquet) would tend to allow the window to act as a frame around a still-life, drawing the eye up to the table top and the view beyond.

Just a few thoughts to consider. I'm sure whatever you decide will work fine. With a view like that, you can't lose!