RE: Cheap rentals in Isla Margarita
You should get a rental at a center as near to your spot as possible, there is a Vela windsurf centre with starboards right at El Yaque, and so is a Club Mistral centre; i don't know about the prices. Just get such a rental offer that you can change boards, so your dad and family will sail in the morning while the wind is low, then when it picks up you will take a smaller board and go sailing yourself. This way you won't have to pay for 2 packages, just one. It probably costs about 200-250 euro (250-300 usd).
If that's not OK you can try another surf centre...