I'm not sure exactly where it stops being navigable upstream from where it dumps into the San Joaquin, but I only see smaller fishing boats going beyond I5 and even those don't go much beyond that as it quickly turns into more of a creek than a river outside of the spring when water is being released upstream. You can definitely go as far as the Stockton Yacht Club, which is as far as I've gone and probably about a mile upstream. There is currently a big barge under the I5 bridge driving pilings for a bridge widening project blocking the middle 50% of the river. It is an interesting side trip, but it isn't a long cruise.
