What about the strain the engine is taking the entire time. As with anything with a load it will start to heat the engine up more than its been designed for. IMO I don't think a 3 person ski is designed to take the load of 1+5 and a hull almost 3x the size.
Such as a minivan carrying a large trailer, sure it COULD do it no one doubts that, and the direct result is that it would be slower. But the transmission is going to heat up and you're putting added stress on the engine that it wasn't designed for, unlike a large truck which has been designed for exactly that load.