A path planning method for an autonomous marine vessel comprising a tow which comprises a sonar module, the method comprising: obtaining an over-ground path of travel for the vessel; obtaining drift conditions for the vessel travelling along the over-ground path; determining a through-water path which represents the vessel path through water when the vessel is travelling along the over-ground path under the influence of the drift conditions; determining whether the through-water path is navigable within motion constraints of the vessel; and providing the over-ground path as the path for the vessel if the through-water path is navigable within the motion constraints of the vessel; wherein when the through-water path breaches one or more of the motion constraints of the vessel, the method further comprises: determining a revised over-ground path; determining a revised through-water path which represents the vessel path through the water when the vessel is travelling along the revised over-ground path under the influence of the drift conditions; determining whether the revised through-water path is navigable within the motion constraints of the vessel; and providing the revised over-ground path as the path for the vessel if the revised through-water path is navigable within the motion constraints of the vessel, wherein the motion constraints comprise a minimum turn radius and/or a maximum rate of turn through the water. obtaining an over-ground path of travel for the vessel; obtaining drift conditions for the vessel travelling along the over-ground path; determining a through-water path which represents the vessel path through water when the vessel is travelling along the over-ground path under the influence of the drift conditions; determining whether the through-water path is navigable within motion constraints of the vessel; and providing the over-ground path as the path for the vessel if the through-water path is navigable within the motion constraints of the vessel; wherein when the through-water path breaches one or more of the motion constraints of the vessel, the method further comprises: determining a revised over-ground path; determining a revised through-water path which represents the vessel path through the water when the vessel is travelling along the revised over-ground path under the influence of the drift conditions; determining whether the revised through-water path is navigable within the motion constraints of the vessel; and providing the revised over-ground path as the path for the vessel if the revised through-water path is navigable within the motion constraints of the vessel, wherein the motion constraints comprise a minimum turn radius and/or a maximum rate of turn through the water. determining a revised over-ground path; determining a revised through-water path which represents the vessel path through the water when the vessel is travelling along the revised over-ground path under the influence of the drift conditions; determining whether the revised through-water path is navigable within the motion constraints of the vessel; and providing the revised over-ground path as the path for the vessel if the revised through-water path is navigable within the motion constraints of the vessel, wherein the motion constraints comprise a minimum turn radius and/or a maximum rate of turn through the water.