A mounting structure for cables for a boat propulsion unit that enables easy insertion of cables into a pass-through hole, as well as highly reliable water-sealing is provided in a bottom cowling of the boat propulsion unit. The cables are inserted in through-holes in a rubber grommet by opening up a slit in the rubber grommet, and then inserting the rubber grommet in a pass-through hole in the port section. The rubber grommet is fit into the pass-through hole in the pressurized condition to water-seal the pass-through hole. The pass-through hole is molded integrally with the bottom cowling. A retaining plate is wrapped around the rubber grommet to prevent the slit from opening up, and the grommet is accommodated in the pass-through hole together with the retainer plate. Then, the retainer plate is fastened to the port section by mounting bolts. Thus, the rubber grommet is fixed within the pass-through hole, and provides water-sealing by filling the gap around the cables within the pass-through hole.