I'm a big fan of using an incident meter.
If you are able to place yourself in a position that "sees" the same light as your subject, the reading you take will, IMHO, will be the most likely to assist you.
If you take a reflected light meter reading, you will always have to also analyze the acceptance angle of the meter and the reflectivity of the subject before you can reliably use that reading.
In the situation you describe, in order to take a reading off the trees, and just off the trees, you will either need to walk across the water or use a spot meter accessory to limit the angle of view of the meter. Alternatively, you can meter the entire scene and then, using judgement and your experience, estimate how much the reflectivity of the scene differs from the standard 12-18%, and adjust accordingly.