When presented with this challenge, my muddled mind conjures a ship in response. It's the one that Suzuki-roshi said life is like setting sail on when you know it's going to sink. Maybe this raft doesn't. Drowning doesn't seem to be the type of stream-entering referred to here. And with the next thought, I wonder if there's no raft at all and if the green land and the arid land are the same, and have different levels of precipitation.
Another stream of thought starts with the raft as our practice. In my darker moods, I think I'm too stupid to do more than sit on the shore and just stare at the raft. "Yep, there it is." Other times, I'm convinced that I'm on it and I could happily propel myself along forever. Both notions are transient.
Thinking lets one know the raft is there, but can prevent one from whole-heartedly casting off, completely confident that the raft floats.
Specific image from about a week ago: my friend stands in the canoe on the tranquil lake, practicing casting with his fly fishing rod and reel. He does this in the midst of chronic pain, unconcerned about catching anything. He said, "It's probably too late in my life to get very good at fly fishing, but I'd like to get better at it."