An annual waterfall trip sounds like good fun! It sounds like you're already well equipped with gear. I'm not sure of what benefit a tilt-shift lens would give you, unless you perhaps wanted to use it for panoramas (by shifting between shots), or just wanted to play with it on other subjects (which is always fun). Otherwise I'd just go with a wider prime, which would likely be cheaper to rent. If it turns out the lighting isn't ideal for your particular waterfall (for example, too much bright light), I'd suggest using HDR. I'm sure you already know that but I was impressed at how much it helps. I'd also suggest trying some night shots. I have yet to do that myself but I've always been impressed by photos of waterfalls with starry skies.
Happy shooting!