Sorry, I will comment now on what you actually asked for!
First of all, I think for anyone besides the people who supported him to erect any sort of memorial would be crass, like rubbing salt into the wound. Perhaps mention of the timeline could be made at the Ground Zero memorial, but it would need to be carefully worded to not enflame resentment or make us seem as if we are glorifying his death. (We are already doing that on TV for the world to watch, lets not make a permanent reminder.)
Second, as far as I am concerned his followers have every right to create a memorial to him. Yes, it probably will become a rallying point and a symbol for them, but nevertheless they have that right. Not that it really matters anyway in this digital age, when they can easily rally and build symbolism through images online.
Thirdly, and this is the part of your question I find interesting, I think a burial at sea has two possibilities from a design standpoint. You can either create a floating monument, which if it can be seen from land is a nice idea, or you can create a monument that is on the nearest shore or the shore of the country where the person was from or lived.
As for memorials for bad guys in general, I think it depends on what you are trying to achieve. The Holocaust memorial in Germany is not a memorial to Hitler, it is a reminder that we should not forget this period n our history and to not let it happen again. That’s very different from a memorial to a bad guy. I think in general I would not be supportive of such a memorial, for various reasons.
Just my opinion.
