How do I boost a wifi signal to my backyard?

Hi. We have a guest house in our backyard that we rent out. We have our internet modem and wireless router in the main house at the closest possible position to the back of the house near the guest house but the wi-fi signal in the guest house is VERY bad. The person we’re renting the place too is VERY inconvenienced and we agreed to provide internet so I’m in a jam. You have to cross a little creek to get to the back guess house which makes it hard to run a wire. I have been looking online at ways I can boost the signal so that the guest house gets it well and I have read about wi-fi repeaters but it is unclear to me whether the wifi repeaters need to be hardwired into the modem. Is there any method that exists that can take the very weak signal and just make it stronger I the guest house? Without a wire being ran?