Final answer:
Motor and water carriers primarily interact through competition, as they both transport goods but have different advantages. Motor carriers offer speed and flexibility, while water carriers offer lower costs for long-distance transport.
Step-by-step explanation:
The main interaction between motor and water carriers in most cases is competition. Motor carriers, such as trucks, and water carriers, such as ships, both transport goods from one place to another. While motor carriers offer the advantage of speed and flexibility, water carriers offer the advantage of lower costs for transporting large quantities of goods over long distances.