Brass and Bronze are terms used very loosely. But in a general sense, "brass" tends to be an alloy of copper and zinc, often with some lead content. This type of mix does not fare well in water, especially salt water, as the zinc is reactive and tends to dissolve, leaving a no-strength copper sponge.
"Bronze" tends to be a mix of copper and tin, and is both stronger and less subject to electrolytic corrosion. Stronger, often proprietary, alloys have nickel and chrome, etc, and get expensive.
So hopefully people are not making brass props except as decorations. Many people have made the mistake of fastening a boat with brass rather than bronze screws. Brass is fine for decorative items, low pressure fittings, etc.