A navigation assist system having a plurality of navigation assist apparatuses installed on small boats like motorboats each fitted with an outboard motor to be able to navigate in a predetermined water area, and a server placed on land to communicate with the navigation assist apparatuses. Each of the navigation assist apparatuses detects a sharp turn position of the boat, adds the position to a danger level data downloaded from the server unit, and alerts an operator concerned when the boat approaches a region whose danger level is great.