The answer is the context.
For the 2nd sentence, you can say it the same way as the first sentence, OR you can emphasize the word, 물 (water), and then add 이예요 (is) separately.
Well, the 2nd sentence isn’t wrong, either. You COULD say 한 손으로 손목시계를 차는 것이 어려워요.
Although they have the same meaning, I think the nuance is slightly different.
1st sentence: 한 손으로 손목시계를 차는 것은 어려워요. “It’s hard to put a wristwatch on with one hand.” The speaker just wants to say this and that’s all she wants to say.
2nd sentence: 한 손으로 손목시계를 차는 것이 어려워요. “It’s hard to put a wristwatch on with one hand…” Here, the speaker is saying this sentence to explain something else. Like 한손으로 손목시계를 차는것이 어려워요. 그래서 매일 남편한테 시계 채워달라고 부탁해요. (It’s hard to put a wristwatch on with one hand, so I always ask my husband it put it on for me.) Or better yet, you would say it this way. “한손으로 손목시계를 차는것이 어려워서 매일 남편한테 시계 채워달라고 부탁해요.” “Because it’s hard to put the wristwatch on with one hand, I ask my husband to put it on for me everyday.”
아이는 신발을 벗는 것이 쉽지 않다. It’s not wrong per say, but it’s awkward, and it doesn’t sound fluent. It can be used when you are talking about a specific child who has a disability or some type of issue. So you can say “이 아이는 장애때문에 신발을 벗는것이 쉽지 않다.” “Due to her disability, it’s hard for this child to take the shoes off.”
아이가 신발을 벗는 것은 쉽지 않아요” “It’s hard for a child to take the shoes off.” This is a general sentence about children.