Partnerships are partnerships regardless of whether it is marriage, business, or sharing a boat, the basic rules are the same. Some partnerships work, some don't. What's important are the people involved and the rules. And don't forget that all partnerships are Wonderful In The Beginning! You go through the honeymoon phaze and then when that's over you are standing there asking yourself "Okay, What now?"
Some things to consider:
1) Usage versus costs: Determine up front and in writing what the rules of use are and how the costs for that use will be split. Also consider alternative scenarios in advance so if issues arise there is already some kind of agreement in place as to how it will be handled. For instance, life circumstances can change. What happens if you or your partner are suddenly only able to use the boat a fraction of the time while the other goes out all the time? Scheduling is a great idea but what happens if the weather for your week makes it impossible to get out? Do you forfeit the week or move it to another time slot? If you and your partner have scheduled use for the year, how do you get another slot? What if you just can't get out as planned but buddy still can? Does he switch time with you, or does he just get extra time and how is this handled?
2) Parachute clause: Ensure that there is an agreeable way out of the arrangement for both of you, again IN Writing! No-one has a crystal ball and friendships can come and go. What happens if one of you is forced to move away due to work, or as a worse case, what if one of you dies? How is the boat to be handled by the estate? If you're young you might poo-poo this idea, but there are a lot of ways to die besides old age. Be prepared. Always have life insurance in place for both parties that will either pay out the partner or pay off the estates portion so the boat can be transferred to a single owner. Take some time to make a list of as many possible ways things could go sideways and make sure you have a plan to get out of them in advance.
3) Get it in Writing! ; Verbal agreements are a disaster waiting to happen in partnerships. No matter how good a friend buddy is, for both of your sakes, and any other potential stakeholders that could end up involved, draw up a written agreement and stick to it. Leave the back-end open so as you both think of other items that could arise you can add them as adendums to the original agreement and initial them. If either of you is offended by this idea then I would not get into the partnership in the first place. Consider this as a Pre-nuptual agreement over the boat since in a way you are married to the other guy once your cash is in the pot.
Take it for what it's worth, but don't come crying if it ever blows up on you.