Go for it! if it's a tradition that you must keep your gown, or if it was really darn expensive and you want to resell, I say get a second dress! You CAN NOT be stressed or worried about such a carefree, fun photo shoot, so buy an inexpensive, or used dress, I know I wish I had for my second, third, or fourth gown (i obviously had issues!)
It will be so wonderful, and fun, so my advice is to also get something that won't be so cumbersome when wearing it while wet, not sure what you currently have!