Steps are detected by an accelerometer. Unfortunatelly, waves in some cases can trigger very similar patterns of acceleration like steps do, so the only way to avoid the steps counting is turning off the watch. Some Garmin watches turn off the step counter during certain activites - for example Instinct does not count steps while biking, so you could also try to find out whether your model has that option too. In such case you could create a custom activity "boat" where the steps counter would be turned off.
