Autonomous surface vessels operating in urban waterways must coordinate with other vessels in confined spaces while complying with traffic regulations, often without knowing their intentions. We present a decentralized motion planner based on model predictive path integral control, a sampling-based model predictive control scheme. The planner jointly computes a trajectory for the ego vessel and a prediction of the trajectories of the other vessels, and therefore accounts for their interaction. Traffic regulations are encoded as discontinuous costs, which the sampling-based formulation admits without a smooth approximation. We evaluate the method in maps taken from real sections of Amsterdam's canals, in encounters between two vessels and in scenarios with four interacting vessels. Across 780 randomized runs the method reaches every goal without collisions or deadlocks, and outperforms an optimization-based planner designed for the same domain, as well as a version of the proposed planner that does not account for the interaction. We further validate the planner on two scaled vessel prototypes in an outdoor environment, where planning and localization run onboard at 10 Hz, and head-on and crossing encounters are resolved without collisions or rule violations.

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