Without knowing more, it's difficult to make sweeping statements here, I feel.
Like demented, I have occasionally sent my oldest son to bed with a sandwich for supper - enough food so he is not hungry, but no treats. Also, if my son won't eat a supper I know he usually likes (ie he is being difficult) I offer bread and butter as an alternative. As my son has a large appetite, loves food, and is certainly no lightweight, I have at times said no to his requests for food between meals - even healthy stuff like sandwiches. And that's not when I have been at all upset with him.
My son had a bad time with his swimming teacher and it did get to me. In our case the blame was on both sides. However, I found the whole thing very upsetting - I sacrificed much time and energy taking him and collecting him from swimming, and kept drumming into him how important it was for him to learn. It's not like learning a musical instrument - learning to swim has important safely implications. I got really frustrated with him for messing around. In fact, the whole thing got to me so much that when my son had grommits fitted, I used that as a convenient excuse to stop swimming lessons. My son is very happy with that decision. He can swim a bit but not really enough. so we will have to go back to it.
Dunravin, I don't know how keen your son is on learning to swim. I don't know how your son usually behaves with his teachers. I don't know how good the swimming teacher is - my son had a temporary seimming teacher who was awful, insulted me and lied to me about my son's bad behaviour (I even started a thread on mumsnet about it). Her version of events was later refuted by other adult witnesses.
I do know that if you hated depriving your son of his supper, and did it this once in the heat of the moment, that certainly does not make you a bad mother.