Sure power comes into play but most ski's large enough to tow also have enough power to handle it at a moderate 30mph or so (being "able" and doing so with comfort are very different). You do need a 3 seater, by law, to tow someone. You need a driver and a lookout (someone sitting backwards watching the person on the tube). Plus, you need room on board for the person in the tube. It's a pain to tube behind a ski, get a boat if that's what you want. Especially with young kids.