Well considering a lake isn't going to have any objects in the way to block signal, I would say just buy an extended range N wireless router, make a directional antenna out of two coffee cans (google or youtube the instructions, parts you should be able to find at radio shack and definitely online) and point it out over the lake through a clear window. Problem solved. You would be surprised how far radio waves travel over water, you might not even need the external antenna (if it's mounted outside). If you need to mount it outside closer to the lake I would suggest building a plexiglass enclosure on a pole and running Cat-6 and power through PVC pipe to the location.
Building materials and cabling ~$100