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I wouldn't give up just yet. Once a boat has been positioned correctly and secured to the trailer with tie-downs it should not move very much. With Boston Whalers some care is needed when tightening the tie-down adjustment due to the sandwich hull construction. Too tight can result in some malformation.

If your trailer lacks keel rollers you may want to consider adding them. The self centering type work well to keep the boat aligned properly. Side bunks may also be useful and guide poles can help, too.

If you post some photos it will help others to help you.