When you walk from SSR to DTD, the first part of DTD you get to is Marketplace. From that entrance, you walk through the Marketplace, then walk through the former Pleasure Island, and then finally get to West Side. When you're standing at the water taxi dock next to Capt. Jack's and Rainforest Cafe, you can look across the lake at the Cirque theater. It's a straight line in terms of seeing it, but you can't walk a straight line between those two points because of the lake.
it's not a terrible walk, you can do it in 10 minutes if you're healthy. But for someone who's looking to minimize walking, the water taxi is a neat option.
But to answer your question in another way, the Cirque theater is as far from SSR as you can possibly get and still be in the DTD complex.