(I just had an idea, and a quick search showed the word is not in the book, so here goes.)
A seastead, with its large size and inhabitants on the top (lighting at night) may easily resemble a lighthouse. Sure, if it was completely dark, it could be an (unlikely) risk to boats without sonar, but this thought seems more interesting.
There are surely laws and customs, and local knowledge, that may enter the picture. Lighthouses have specific light colors and frequencies of blinking, you will want to be sure you don’t resemble the way to safety for someone. Besides the potential hurdle (“Fishing boat sinks after mistaking seastead for a lighthouse, three dead.”), there’s an opportunity to gain goodwill as well – what if your seastead, as long as anchored in a certain place, would function as a lighthouse for the locals – if the energy costs are acceptable? Bonus points if it happens to be anchored on shallows or another area risky for boats.
A lighthouse can´t move. So you´d be giving up the dynamic geography feature of seasteading. Wouldn´t you have to commit to at least a couple of decades of lighthouse service for this to be somewhat worthwhile? Besides is there really a lighthouse shortage?
First, the primary purpose of a lighthouse is to warn people off a hazard, not to draw them in, and only secondarily to provide a navigational beacon.
Second, lighthouses are pretty much obsolete, abnd being shutdown everywhere, as the expense/benefit ration has declined drastically with the availability of cheap consumer electronics for everything from GPS to radar.
Third, those who are more likely to gain benefit from a lighthouse or be confused by something they mistake for a lighthouse, are not likely to be where you are at: namely, small pleasure craft, and the middle fo the ocean.
Fourth, you are probably night shining the same kind of beacon light, but rather have lights much like the running lights of a ship or oil platform.
Benefits-wise, I think that you could probably gain a small bit of revenue from operating as a navigational aid by being a stable reference point for radar, by broadcasting on a frequency which could be used for direction finding (although GPS has mostly eliminated a need for this), and as a meteorological station. If you were to moor close to an open-ocean navigational hazard otherwise unclaimed, you might be of service as a warning buoy. If you were located within AM or FM broadcasting range of shipping routes, or small islands, you might make some ad revenue as a television/radio broadcaster.
I see these things being possible, because unlike some others, I don’t see a free-drifting platform as being feasible and desirable in most cases.
Now, that I call constructive critique: note the weak points of the thought, but list useful ideas related to it – thanks thebastidge! The meteorological station and warning buoy are both very possible, and a radio station could bring some PR.
Aside: does anyone have some resources on shipping routes and the like? Situating something close to a few would be profitable, as noted.