What about Google Latitude? I don’t know how fine the location resolution is, I’ve never used it but it does have a pc version so your Dad could use it from home. And it’s free AFAIK.
As an aside: A smartphone seems like an odd choice for the job. Can you power it from the jet ski (the GPS will suck a lot of power)? Can it get a good GPS reading from its mount? Doesn’t it get wet? If you should become capsized or such, will it float and will it still work?
If stuff like that matters, consider something like the SPOT Personal Tracker instead, specifically designed for personal location tracking. It includes a one-button SOS mode if you need help, no waiting for Dad necessary.