imagine : since tin man can’t really come in contact with water because yk he will rust and become immobile, he would 100% still miss the feeling of jumping into a body/bodies of water. Due to this, he expresses his feelings, telling the group that he really misses being able to go in water, especially submerging in it.
Scarecrow listens and feels bad. So, to make him feel better, he will become a sacrifice, literally becoming a surfboard/barrier so Tin Man can like hover over water without actually getting in and coming out rusty. Just when they’re about to do this, like in position and everything, Dorothy swoops in and IMMEDIATELY SCOLDS THEM; explaining to them that scarecrow WILL SINK AND LITERALLY DROWN BOTH HIM AND THE TINMAN. This is what having no brains and yet lots of empathy for a friend does to you