The OP asked two questions: if liability only policy is wise for a sub $50K hull, and if so, how much liability would be enough.
My answer to first question is yes, and the answer to the second question depends on the financial position of the owners. If neither couple has much in the way of assets, you have less to fear from a lawsuit and less need for higher limits. Why? Because no attorney will take the time to sue two couples who have no reachable assets as the attorney will not get paid. In that instance, any party damaged by your boating will from a practical standpoint be stuck with your policy limits.
Our marina requires a minimum of $300K in liability. Check with marinas in your area, take a look at what you have to lose if things go wrong, and insure appropriately.
As was mentioned above, your liability is not generally limited to the 50% ownership. Let us imagine you and your wife have assets and the other couple does not. "Joint and several liability" will enable a potential plaintiff to have a go at you for the entire damages in excess of the policy limits even if you were not on the boat when the harm occurred. Choose your co-owners wisely as well.