Your marina may require liability insurance. What it will cost will depend more on how you are rated (no prior ownership; boating safety course or formal training? etc.) than the value of that boat. If you have auto insurance, many companies will cover boats up to 28-30' at a bargain rate since it is just another motor vehicle on your liability policy to them.
Boats since 1972 have been built with a HIN, similar to a VIN, and usually are titled and registered similar to a car. Since you are looking to buy anyway...stop by some local brokers and ask. You might find one worth dealing with, and a boat to go with it.