No. There is absolutely no charge.
You will receive a tax deduction for the amount the boat sells for.
If the boat sells for under $500, you will receive the value of the boat up to $500.
Kars4kids is a classified 501(c)(3) charity organization. You can check with your tax advisor or accountant to determine the amount of your benefit. The deduction is of value only if the donor itemizes his or her deductions.
For a more detailed overview the IRS publishes a Donors Guide to Vehicle Donation.