Waterproof boat covers that only protect the boat from the elements really aren't appropriate for boats that are in the water. They only extend a couple feet from the hull, and are designed to keep the boat free from leaves, dirt, bird droppings, and other debris, which makes sense for boats that are in storage. If your boat is going to be in the water, then you need a cover that has the capability to absorb water and repel water, not just keep the boat clean. This is why the boat manufacturer offers a cover, and why they are much more expensive than the covers that are made simply to protect the boat from the elements and keep it in storage.