Most of the Bimini's are made of Sunbrella. This is generally water proof, but does "breath" to some degree. The proper attachment to the cabin top is a vinyl or aluminum awning rail, slotted track and a bolt rope slides into it. There a number of non permeable fabrics which if properly assembled are totally waterproof, but do not breathe and thus are more likely to develop mold because of high humudy on the water. This cabin to Bimini top seal, usually involves zippers and velcro over to make it totally water proof for most rain events. Hurricanes: all fabrics outside come off the boat.
Fabric such as Sunbrella, do fine. Others non marine fabrics do not do as well. See SailRite web site for other materials. Would I leave my living room sofa there? No.
If you are speaking of a full "camper back" Again it depends on materials and how they are made and correctly fitted to the boat. A good camper back may cost upward of $6,000. The canvas maker should fit it for your specific boat and needs. There are many configurations.