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If the boat WAS sailing normally, and you didn't hit anything, then I'd doublecheck the rudder. Have someone visibly center it, then see if the wheel is centered. Sometimes it is that simple, something in the steering linkages has jumped and needs to be recentered.
Otherwise...you need to eyeball the underbody, make sure the keel wasn't shifted during a grounding or previous repair, check the rudder for delamination, swelling, or possibly being bent. When you get an eyeball at the right angle to it, you'll see something isn't symmetric below the waterline.