Composites will generally be lighter than metals for the same stiffness/strength. There are a number of problems with using metals on such small boats, mainly that it would be difficult to weld the thin sheets and maintain a fair hull. They'd also be suceptible to looking like a starved horse over time.

Also, have you seen a standard tinny after a couple of years of abuse, they are significantly dinted, probably more so than an equivalent weight cored composite construction tinny. A "durable" plate aluminium 5m tinny will weigh significantly more than an equivalent composite boat, but I wouldn't run the composite boat onto the beach at speed unless I didn't care about destroying the bottom.

From my understanding, titanium is very difficult to weld. You can't just use a standard welder with a gas shield, you need a full inert gas room. It has been done though, apparently the Russians built a couple of titanium submarines, they may've also been double skinned.