This answer is generic since I don't know where you are. You and "partner" are likely both listed on the title currently. Draw up a bill of sale
stating the interest being purchased and the price
. The title will generally have a spot on it for the seller to sign away his interest. Go to your jurisdictions' registry or here in Washington
it would be the Department of Licensing with the title and bill of sale
. Here (in WA.) you would pay tax on the 1/2 interest that was purchased and sold. Easy.
You didn't mention that the boat was owned by an LLC so I will skip that for now.