KenV is correct - they did use conventional bombs (tho I suspect a lot of people got as low as they could and dropped the bombs early and claimed it was deliberate skip bombing when it worked)
Skip bombing and mast height bombing are similar and sometimes used together. Mast height bombing was generally done at similar altitudes, but at higher speed (265-275 MPH) than skip bombing. When used together, the first bombs were deliberately released early to skip across the water and strike the sides of the ship, and the following bombs were released later to drop directly onto the ships.
Skip bombing required different fusing than mast height bombing. Skip bombs required delayed fuses and mast height bombs required contact fuses. If a delayed fuse bomb was used for direct bombing against unarmored transports and merchant ships, they would pass through the ship before detonating. If contact fused bombs were used to skip bomb, they would detonate when they first struck the water. So the crews could not "accidentally" do one when they were trying to do the other.