I will assume that you are talking about a pleasure boat -- not a cargo vessel, for-hire touring charter or a for-hire fishing charter. If there is going to be _any_ commercial activity with the boat whatsoever you would need an attorney well-versed in maritime law to sort out the issues of liability, master of the boat, and the like. Luckily, for personal pleasure boating the answer is relatively straightforward.
First, there is very little in the world that can be done to avoid being sued. If somebody wants to name you as a defendant it is very simple to do and essentially cannot be stopped in advance. That being said, how long you stay in the case will depend on the facts of the situation which led to the suit. Whether you have to pay anything in such a suit -- other than your defense costs which can be substantial -- will depend on the facts of the case and the facts alleged in the suit.
Second, generally speaking the person operating the boat is responsible for the consequences of that operation. Simply by being co-owner you are not going to be liable for whatever the other person(s) might do with the boat. There are exceptions, especially if you know what they are up to and don't stop unsafe or illegal things (no life jackets onboard, a persistent fuel or oil leak from the engine, they suddenly start chartering the boat for deep sea fishing or running drugs across the Columbia).
Third, you should insist that the co-owners have their own insurance policy specifically covering the boat. If you are the only one with insurance on the boat, you will absolutely get sued if something goes wrong. If you have the only pot of money sitting around you can guarantee that somebody will try to go for it, with creative pleadings or some theory of negligent entrustment, but you do not want to be the only ones with insurance on the boat. Insurance coverage draws attorneys like flies to honey.
Finally, it might be a good idea to sign a contract with a buy-out provision. That way, if either side gets scared of what the other side is doing, you can sever the connection and stop any prospective liability. This won't protect you from something which had already happened, but it would cover you going forward.
The best way to protect yourselves (assuming this is a pleasure craft and not a for hire craft) is to create a corporation or LLC and transfer your portion of ownership into the new corporate entity. This will insulate you. Of course the corporation will need to conduct some kind of business to be legitimate, even if a non-profit.
You should consult with maritime lawyer (may need to refer to San Francisco or Seattle). Most maritime lawyers know about corporate matters and could assist you further there, as well.
Adam Jaffe Law Office of Adam Jay Jaffe 124 Lomas Santa Fe Dr, #204 Solana Beach, CA 92075 (619) 810-7964 www.smallclaimsappeals.com Adam@SmallClaimsAppeals.com This posting is provided for “information purposes” only and should not be relied upon as "legal advice". Nothing transmitted from this posting constitutes the establishment of an attorney-client relationship. Applicability of the legal principles discussed here may differ substantially in individual situations or in different states.