Sunk boat does not equal abandoned boat. It depends what CaRT classes as abandoned. Given a number of 200, my guess would be they class abandoned, where they do not know the current owner, or all attempts to contact, or trace the last known owner have failed. A sunk boat could have a known owner. CaRT could be trying to get the money to recover it from the insurance company, or owner, before spending their own.
If it is a wooden hulled boat, then sinking it can sometimes be the best option to slow further deterioration. So a sunk wooden boat could have an owner that has some concern for it's future. See the various deliberately sunk wooden boats at the Ellesmere Port museum, where the numbers exceed the resources available for their restoration, ot even conservation.