Being a college student has its advantages in the ski world. When my college team friends and I go to ski with other people we always offer cash but it is rarely accepted. It is great that people like to help out the college skiers. I will gladly let college skiers ski for free with me when I am older.
That being said, when I invite my college skiing friends to come to my place to ski I expect some form of payment. It is tough footing the bill for boat gas and truck gas when I don't even have a job during the school year.
In reality it is the thought that counts, if your ski partners don't even try to pay, they aren't great partners. And also don't ask the person how much you owe them, give them more than what you would expect to be paid if it was your boat. Its awkward telling someone how much to pay you because you frankly don't want to sound like a cheapskate asking too much.