My boat is much older than yours with an aluminum water tank. Maybe yours is similar? Anyway I had the same issue with pin hole leaks in the aluminum pickup tube. I had to cut an access port in the top of the tank. The rest of the repair consisted of reaching inside with a hacksaw, cutting off the tube but leaving about a 1" stub, then sleeving the stub with a new pick up tube made from ordinary PVC hose and a hose clamp. The port is a standard clear cover gasketed item readily available in multiple sizes. The larger the better for future cleaning of your tank.