Not sure precisely what you're asking. I'm thinking in terms of clearing customs.

Most of the customs requirements I've seen state that the captain must clear customs. Some say captain or owner. I've even seen captain AND owner. I don't know whether, if the owner of a vessel is on board, s/he is presumed to also be the captain unless there is some document appointing another person on board as the captain. It may vary from country to country.

If the rules permit an owner to clear customs (either explicitly or implicitly), one option is to make you and the other person concurrent owners. There are three types of concurrent ownership in the U.S.: tenancy in common, joint tenancy, and tenancy by the entirety (only for married couples and not allowed in all states). There are legal differences among these three types of concurrent ownership (which I will be glad to explain if you want or you can look up.) The important thing to keep in mind, however, is that once a person is an owner, s/he has ownership rights which includes (except in the case of tenancy by the entirety) the unilateral right to transfer his or her interest in the property.

The other option is to draft a durable power of attorney giving another person the authority to control the vessel in the event of your incapacitation. Note that a general power of attorney automatically ends when the person who gave the power of attorney becomes incapacitated. Only a durable power of attorney continues in force after incapacitation. A durable power of attorney can be limited in authority or it can be written so that it only takes effect when the person becomes incapacitated.

If you are thinking of giving another person control of your vessel only in the event you become incapacitated, a durable power of attorney is the approach I would use. But it may not eliminate all legal hassles. Before recognizing it, a country may require proof that you have become incapacitated. And if you go this route, I would make the document as formal and official looking as possible. In my experience, seals, notarizations, and other bells and whistles tend to impress customs officials.