If the screws are below water level you will need to use manual tools. There is a tool by name of easy out that is use to remove broken screws. If any of the screw head is above the ring, vise grip locking pliers work well. Now if they are above water the screws can be just drilled out. Use a drill bit smaller than the screw diameter. The you will possible need to retap the threads for new screws. The new screws sholud be stainless steel.