Nothing is 100% safe, who said Nuclear power is? Nuclear power always has been and still is one of the safest ways to generate electrical power and this is what counts. Solar power for example causes a lot of fatalities due to accidents that happen during installation and has it's own environmental problems, yet nobody talks about it.

What if they build this on a floating oil rig type situation and fallback dumps it into an ocean fed pool (like a box with gaps for ocean water going down 100ft) far enough off the coast to have power lines but easy enough to recover?

Nuclear power is comparable in safety to solar and hydropower. https://ourworldindata.org/safest-sources-of-energy

Watch the "A is for Atom" episode of Adam Curtis' Pandora's Box series. It's linked in one of my other comments. Basically industrialists and sales people got involved, brushing off scientists' concerns. And irresponsible politically connected engineers in the USSR. That's why we have Chernobyl and Fukushima.

However, these are lessons learned. Today it's quite hard to come up with a reactor design without many safeguards and passive safety features, like it's quite hard to come up with a car with no seatbelts or airbags.