Here are my thoughts. I never expect money from someone I ask to come out boating. If they offer, I decline twice, and accept on the third attempt. What is really tough is when you accept and then they ask 'How much??'

Think about it. We typically go through maybe 6 gallons per hour. If you do not include drive time to your site, which can be substantial on some lakes, and just look at stick time per person, a typical skier may go through a half hour worth of gas. So we are talking 3 gallons. Unless you are buying marina gas, should be less than $10.

If you asked the couple over for dinner and drinks.
You might say 'Bring over some steaks and we can BBQ.', OR 'My bar is not well stocked so you might want to bring over your favorite..'.
Would you say - 'We are having steak so bring cash.'

If no stipulation is made during the invite, results can vary. But $10 is barely more than a drink at club these days. Is it worth the anxiety? In all honesty, if the guys is being brought along as a friend-of-a-girlfriend, then the girlfriend is more liable than the guy.

If the shoe is on the other foot, I would always volunteer money or gas if I were the guy. But I know some people who get offended if you offer money.

I would say - OK to be a little annoyed, but don't let it bug you. Unless the guys is burning through $30 worth of gas...