Of course it's cheating if it's against the rules. The question is whether it's just cheating oneself, or everyone.
I've only had one real disaster of a race. Early last year I did a race (non-standard but a bit tougher than a typical 70.3). The swim was the root of the problem I think. I hadn't been doing much swimming in the lead up but should have been okay. On the day, I swam considerably slower than usual and felt awful. It was a relatively small but a strong field. I'm used to finishing in the top half, I came out of the water second last. It was two 1km loops and I already felt awful after the first. The leaders were already heading for their bikes when I ran across the beach and back into the water. The bike had a turnaround and a very aggressive cut-off time, not aided by a strong headwind on the outbound leg. I passed a handful on the bike but still missed the turnaround cut-off, by about 2 minutes. I asked the offical at the turn if I could finish the race and he said yes, so long as the sweep car didn't catch up to me I could continue but I wouldn't be included in official results. That was fine with me. I carried on, still feeling awful for the remainder of the bike, managed to overtake a couple more cyclists and stayed ahead of the sweep car. I started the run and did half the first loop of a 3 loop course before severe knee pain forced me to walk. I still started the second loop, hoping to get moving again but already knowing the result would be a disaster no matter what happened. By the far end of the second loop I was limping badly and just about made it back to the turn for the finish line but with one lap not done. I took the turn, got cheered as I approached the finish, which I found very embarrassing, and then I stepped to the side and walked down the outside of the finish chute to find some friends I knew would be waiting at the line. A volunteer still chased me with a medal which I politely refused.
I could have skipped the second lap of the swim, though it might have been obvious. I could have turned early on the bike route, which would have been easy and probably wouldn't have been seen. I could have taken a short cut on the return leg on the bike rather than do the very substantial 10km+ climb with nearly 1000m of ascent. I could have stopped on the first lap of the run rather than the second and could have pretended I'd finished as I went down the finish chute. It's unlikely anyone would have noticed. My training buddy who I regularly beat was already finished having done the full distance, by the time I reached the line having cut it short. I only stopped early because I was worried I'd damage my knee further if I didn't. I only continued because I was permitted. I only went to the finish because I had to meet people. I was kinda proud of hanging on as long as I possibly could despite a truly horrendous day that I have never been able to explain.
If you risk missing the swim cutoff, try anyway and see what happens. If it's so bad your safety may be at risk - pull out. If you can't make it all the way, go as far as you can....within the rules. Making a calculation and deciding to move onto part 2 without finishing part 1 is a different thing entirely.
I've only had one real disaster of a race. Early last year I did a race (non-standard but a bit tougher than a typical 70.3). The swim was the root of the problem I think. I hadn't been doing much swimming in the lead up but should have been okay. On the day, I swam considerably slower than usual and felt awful. It was a relatively small but a strong field. I'm used to finishing in the top half, I came out of the water second last. It was two 1km loops and I already felt awful after the first. The leaders were already heading for their bikes when I ran across the beach and back into the water. The bike had a turnaround and a very aggressive cut-off time, not aided by a strong headwind on the outbound leg. I passed a handful on the bike but still missed the turnaround cut-off, by about 2 minutes. I asked the offical at the turn if I could finish the race and he said yes, so long as the sweep car didn't catch up to me I could continue but I wouldn't be included in official results. That was fine with me. I carried on, still feeling awful for the remainder of the bike, managed to overtake a couple more cyclists and stayed ahead of the sweep car. I started the run and did half the first loop of a 3 loop course before severe knee pain forced me to walk. I still started the second loop, hoping to get moving again but already knowing the result would be a disaster no matter what happened. By the far end of the second loop I was limping badly and just about made it back to the turn for the finish line but with one lap not done. I took the turn, got cheered as I approached the finish, which I found very embarrassing, and then I stepped to the side and walked down the outside of the finish chute to find some friends I knew would be waiting at the line. A volunteer still chased me with a medal which I politely refused.
I could have skipped the second lap of the swim, though it might have been obvious. I could have turned early on the bike route, which would have been easy and probably wouldn't have been seen. I could have taken a short cut on the return leg on the bike rather than do the very substantial 10km+ climb with nearly 1000m of ascent. I could have stopped on the first lap of the run rather than the second and could have pretended I'd finished as I went down the finish chute. It's unlikely anyone would have noticed. My training buddy who I regularly beat was already finished having done the full distance, by the time I reached the line having cut it short. I only stopped early because I was worried I'd damage my knee further if I didn't. I only continued because I was permitted. I only went to the finish because I had to meet people. I was kinda proud of hanging on as long as I possibly could despite a truly horrendous day that I have never been able to explain.
If you risk missing the swim cutoff, try anyway and see what happens. If it's so bad your safety may be at risk - pull out. If you can't make it all the way, go as far as you can....within the rules. Making a calculation and deciding to move onto part 2 without finishing part 1 is a different thing entirely.