I should imagine that if you lost something you cared about then you may feel differently about that!
This is the position in law as far as I understand it:
Report your find to the local police. You can do this by telephone. Tell them that you are holding onto the dingy and that it will be available for the looser. The police will record your find. If the loss is not reported then you continue to keep the dingy. If someone reports the loss then the police can inform them where it is. If they then ask for it back you must let them have it.
By loosing something the 'title' to it does not pass to the person who finds it.
Do the above and you have both complied with the law AND your moral responsibilities.
Keith (do the right thing) Hart