I'm going to second the Google calendar suggestion. I have loaded a perpetual shift calender for both myself and my partner, so we always know when the other is working. This is incredibly useful because he is a police officer and I'm a firefighter, so we both have schedules that change every week (thought it repeats about every 2 months). There is a seperate overlay (calendar) for "boat time" and we just fill it in on a first come/first served basis, with a spirit of cooperation of course, and trying to work around each other's work schedules and vacation plans as much as possible.
Changes to the calendar can be made by either party and are immediatly viewable from any internet connected computer.
On the partnership subject, I couldn't be more happy with it. It's all about finding the right partner and I place a considerably higher value (both satisfaction and monetarily) on the partnership than I do on the boat. I doubt I will ever own a boat by myself unless it's after I retire and I want to be gone for months at a time.