Like the folks say, the flex option is a great one, and the more you add, the more weight you get. It's the same stuff as is in the truck-lining material, only the primer is specifically for pvc (be sure to tell them when you order that it's for pvc). We've been able to eliminate small leaks with this.
Inflate your boat such that it barely holds its shape, mask off you sections and mix all components in a 50/50 mix. Primer - 50% part A, 50% part B. Stir well, then add as much primer as you have the flex stuff (50/50 again). The smell is super nasty, but hey, free headache with buzz. You have probably 45 minutes before the whole mixture hardens to an unappliccable mess at the end, with it speeding up the curing process at the end. I was really surprised at how quickly it went from being able to roll it on nicely to barely being able to get the roller to separate from the tray towards the end. It's roughly $100 per gallon, which can do an entire underside of a 16 footer *thick*, so purchase and use wisely. Pull off masking tape before you decide to let it dry.
If it's a bigger leak and you live on the west slope, drop by and I'll fix it for you for a sixpack.