Hypoeutectic aluminum-silicon alloy consisting of 8.0 wt.% silicon and prepared through a solid-phase technique was studied for the contact angle behaviour of oil in underwater condition. The surface structure of the Al-8 wt.% Si hypoeutectic alloy was modified through a solutionizing heat treatment process at 650 °C for 6 hours, quenching in water and etching in Keller's reagent. The modified surface structure showed an increase with the etch time for sizes of the secondary arm-spacing (SAS), from 5.31 ± 2.54 μm to 6.16 ± 2.35 μm for 5 to 20 seconds of etching respectively. Equally, while the average surface roughness (R a) increased from 0.1 to 0.74 microns, the sizes of the secondary dendrites (SSD) reduced considerably with increase in the etch time. These alterations of the surface structure influenced the fractional surface area (f so) and the evaluated contact angles of oil drops at the solid-water interface. Therefore, a gradual increase in the contact angles of oil, to a maximum value of 159.96 ± 0.64, was noted for the Al-8 wt.% Si alloys after the solutionizing heat treatment, quenching and etching process, confirming significant underwater superoleophobicity.