When you start, Del Lago is towing you and heading toward the pieces of land in the middle. Steer to avoid them and harpoon him as soon as you can afterwards (before it lets go). Next, aim a harpoon and throw it at any sign of movement so that you will not be knocked off the boat. After awhile it will go away, then charge at the boat. Throw as many harpoons at it as possible before it reaches the boat. If you get knocked out, press X until you get back in. Repeat this process until the intermission sequence starts and Del Lago retreats.