You bought the land for $75,000. Waterfront living goes by the feet on the water usually. Take however many feet you have on the water to determine the price you originally paid per foot. You sold 3 feet, that is your basis. Alternately, you can divide to get the price per square foot. The IRS will accept any reasonable method.
Improvements like a house and boat dock do not affect your basis in the land unless part of the land you sold. Landscaping would increase the value. You are correct that the price difference in the property is significant with waterfront life.
Did you receive a tax form to have to deal with this?
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