Any tour with a limited capacity (small boats, planes, etc.) will book up quickly. I would suggest booking now, but find out what their cancellation policy is to see if it will allow for "wiggle room" if the weather isn't safe (it should!)
You can't let a little rain hold you back from enjoying your time in Alaska... it rains all the time here in SE! If I waited until it wasn't raining to do anything, I'd never get anything done!
Happy Alaska Travels!
Better not for now... signatures get me banned!