Use aluminum, it’s lighter. It won’t last as long but it’s cheaper than fiberglass. FG tends to breakdown from the sun and it will start to shed after a while. Aluminum is compatible with pool water and won’t rust. It may develop a white scale from chloride exposure but it is harmless to the water.
I buy cheapo $30 aluminum umbrellas from Walmart or Ace. They last a few seasons and then I toss them and get new ones.