Every situation is different, (and personal relationships are all different) but because water skiing is different from (say) snow skiing, where you would HAVE to purchase an expensive lift ticket or wouldn't be able to ski, there are a few things to consider......
1. Who "called" the ski date? This is probably the biggest "luxury" that the Boat Owner gets, and it is often an expensive adventure (typically you must OWN or control the boat) to call the ski date.
2. What other direct expenses does the boat owner have, in order to "go ski with you". i.e. trailering the boat to the location, driving out to the lake, etc.
3. The cost of the boat "gas" is only a small percentage of the ski ride. Any boat owner quickly learns about the other expenses (maintenance, licenses, registrations, insurances, trailer, trailering gas costs, the lake, parking and boat permits to use the lake, the cost of a vehicle maintained just so he can pull his boat, etc...) Those costs often serve only one purpose...so we can ski!
If you are lucky enough to NOT own a boat, yet go skiing with a buddy who has one, you are in an excellent situation, especially if you get to call the ski day/time. To only pay "gas" is to not respect the many costs associated with your skiing, and unless your boat owner friend has lots of $$, and just loves entertaining you, you may soon find yourself not getting invited out to ski.
With many things to consider (all relationships are obviously different), I believe a respectful amount to offer is at least $20/set. While it is not appropriate for the boat owner to add up all of the costs associated with the boat, then divide that amount by the amount of sets skied in a year, and charge THAT amount, at least the guest has showed some consideration by "chipping" in for MORE than just his gas costs. I think that this is what @jdarwin meant by his post, of gladly letting someone have $15, and pull him all day long, as he understands the costs of boat ownership.
At the end of the day, are you invited back? Is that important to you? Do you like to "call" the ski date? Is your training something you take seriously? Are you taking advantage of, or are you respecting your "boat owning buddy" and saying "thank you" to him, not only with words, but some green as well? It's a great sport (but yes, expensive)! And as Schnitz once said..."To offer a $5 or $10 bill for a ski ride, is actually an insult" (though I add to that, often not even realized).