Tethering a Floating Lighthouse I bought a 3-foot miniature lighthouse and I want to float it in my pond. My problem is in keeping the lighthouse erect when the wind blows. The attached sketch illustrates the problem. We have the lighthouse glued on top of a 5-gallon plastic bucket (5-gal or so of air), a threaded rod attached to the bottom of the bucket holding 20 pounds of bricks (35 lbs, but it displaces 15lb of water). The top of the bucket is tethered to a weight on the bottom of the pond. At some angle from the anchor to the bucket the tether will counteract the tipping force of any wind. But what is that angle?

 A: Great question and sketch. 
The attachment point of the rope to the lighthouse will be at about 60% of its height above water.  It is the same point at which you would apply your finger horizontally to prevent any tilting due the wind's force, which of course is only above the water surface.
So ignore everything under the water, the wind's forces are distributed along the height of the tower according to whatever its detailed shape is. The nearby vegetation will also slow the wind down closer to the ground, so that's why I guess the "neutral point" of the wind on the tower is at about 60% of its height above water.
The effect of the tension in the angled rope can be broken down into two orthogonal components: horiz and vertical. The vertical component is counteracted by your floats. The horizontal component alone eliminates the tilt, and this is the equivalent of the critical placement of "your finger" described above. So wherever your finger would need to be, that is your rope attachment point.
You don't want it visible....?  Then add on a lever arm under the water surface to attach the rope at the same angle. The effect will be the same as physically attaching it to the tower above the water line!
See your modified diagram attached.

