You basically need a few conditions to be met to make this useable: tide needs to be high enough, there needs to be suitable geological formation that enables building of such power plants, it has to be publicly acceptable to build there, and you need to connect it to the grid. The last two can especially cancel eachother out.

However, this assumes you use potential energy. What you are envisioning might be more like current power (so kinetic energy) where I’m not sure what the limitations are. Perhaps it’s not too practical to build huge plants underwater in locations with relatively constant current and connect them to the grid

Hehe, I walked right into this one. You’re right. I totally failed at trying to be a smartass.

Why are you comparing fossil fuels and nuclear “per tonne” that makes no sense. You replace tens of tones of nuclear fuel per year any you burn millions of tones in a comparable fosil fuel plant.

And regarding the carbon emissions from enrichment… Just use nuclear to power your enrichment plants. This way your emissions are extremely low because you don’t need much fuel and you use nuclear energy to produce nuclear fuel. French example: https://en.m.wikipedia.org/wiki/Tricastin_Nuclear_Power_Plant

With energy positive here I mean useful energy positive, so electricity or high temperature heat.

You are technically wrong, the worst kind of wrong :)

DT and DD fusion reactions release energy. More energy than is put in. It’s the whole system that hasn’t been energy positive. We’re close to breakeven in terms of plasma (heating power vs fusion power, and it’s not like heating power is lost from the system it still heats the reactor) but to be useful fusion power needs to be >10x heating power so the whole system is more than self-sufficient.