49 CFR §39.83

1. (a)If a passenger with a disability can readily get on or off a passenger vessel without assistance, you are not required to provide such assistance to the passenger. You must not require such a passenger with a disability to accept assistance from you in getting on or off the vessel unless it is provided to all passengers as a matter of course.
2. (b)With respect to a passenger with a disability who is not able to get on or off a passenger vessel without assistance, you must promptly provide assistance that ensures that the passenger can get on or off the vessel.
3. (c)When you have to provide assistance to a passenger with a disability in getting on or off a passenger vessel, you may use any available means to which the passenger consents (e.g., lifts, ramps, boarding chairs, assistance by vessel personnel).