Why did the man bemoan the loss of two pounds worth of food after his sea voyage was over?

The man bemoaned the fact his money was wasted, as he'd been seasick for the entire trip. He could have paid for his meals as he ate them but chose to pay in advance.

Another fellow I knew went for a week’s voyage round the coast, and, before they started, the steward came to him to ask whether he would pay for each meal as he had it, or arrange beforehand for the whole series.

The steward recommended the latter course, as it would come so much cheaper. He said they would do him for the whole week at two pounds five.