Tough Mudder is not a race, nor is it about getting a better time than other competitors, so it's fine by us if you want to skip an obstacle and use the bypass lane. The way we see it, you are there to have fun and challenge yourself. If you think a particular obstacle really isn't going to do this for you, just go around it without a penalty or shame.
Anyone who cannot swim (and would have a difficult time in water) is strongly advised to skip these obstacles, as safety is held at a premium at Tough Mudder. Please speak with a doctor before coming to Tough Mudder, as they can evaluate your specific circumstances.