This is a great idea on a good weather day, especially if you like boating, canoeing, or outdoorsy activities. This is especially great if you want something very private, and romantic. This can be planned by using a friend's boat or canoe or renting one out. You can bring music to set the mood, play an instrument if you know how, or even sing to her if you have that kind of ability.
Otherwise, you can just go for the proposal without the other props. You also have the option of having a special picnic after the boating ride. You can propose to her at the picnic as well. The picnic can be a surprise that you already laid out with the food, wine, and music. You can ask a friend to show up and set it up for you close to the time that you will be arriving for the picnic. This will make the day even more special and meaningful!