If the zipper is still good(box, teeth, tape, stitching), and the slide is the problem, that is a pretty simple fix, on the boat. The box is what joins the 2 halves together. Slide the slider all the way to the other end, and remove whatever is keeping it on the teeth. I could be a sewn tab, or one of the stops shown in the link below. Put a new slider on, narrow end first, and put a zipper stop, preferably the second one in the link, on where the zipper stopped before, and you should be good to go. If the end of the zipper is sewn into the canvas, and not readily available, you can pull a tooth with needle nosed pliers, 3 or 4 teeth above where the stop goes, and remove the old slider there, and put the new one on.
If you don't want to order, or are in a hurry, go in Jo-Anne Fabrics, and buy a short YKK #10 Vislon separating zipper, and take the slide off. Practice pulling some teeth on it. A safety pin through the zipper tape, as close to the teeth as possible, works as a stop until you get the right one. So does a dap of epoxy between the teeth.
Don't get a locking zipper for that application.