It's not whether it's against sikhism or not, it's whether you want to protect your own grace. Nature gave men an automatic response to seeing a woman's body. You can argue that it shouldn't be that way, but it is.
If you want to stimulate the lower nature of every man who sees you, feel free. If you want to inspire those who see you, you might have to go to more trouble.
You can wear more modest clothes and still swim, like a tee shirt over a sports bra and knee length shorts, or something similar. You don't have to wear a bikini, nor do you have to wear a chador. You can still have fun and protect your grace.