It's easy. If the boat is small it's likely state registered with the DMV. Take the title in and they'll transfer it into your name and give you current registration
for a small fee. You'll have to pay sales tax (at least that's the case here in Cali).
If on the other hand the boat is Documented, you will not be able to have new documentation
issued in your name, as you must be a US citizen. If this is the case you need to have the old owner contact the documentation
center and tell them the boat has been sold, and then you need to call and have the boat " removed " since you will be state registering. In Cali you do not need proof of this (by way of a letter the doc center can send you for a fee) - other states may require the letter. Once this is done you go to the DMV and they will get you a state issued title and new registration
USCG National Vessel Documentation Center, Home Page