I don’t think that is heartless at all and I don’t see how people would think that. It’s like not going to your favorite restaurant or bar anymore just because another person died there on the same night you happened to be there. I would use scuba diving as a way to pay my respects to the diver that lost his life. Find a still spot on the reef, pause, and reflect. Life goes on man, have fun until it’s your time to go! And forget what other people think.