From what I have read, fluoride’s action on teeth is purely topical. Which is why it is in toothpaste (which is not swallowed). The “minuscule” amount in drinking water is reported as not enough to be toxic, yet somehow enough to strengthen teeth through internal blood circulation. Any fluoride you ingest, even a few atoms, is considered a toxin by the body and removed. So while the minuscule amounts added to water may not harm you, they are still adding to the “workload” your body has in dealing with all the minuscule amounts of other toxins you acquire daily.

Talking about the ocean is odd, but there are towns in the UK (and most countries I'd assume?) where the natural level of fluoride is higher than the concentration they aim for when adding fluoride. I think that's a pretty good argument for it being safe - the people of Hartlepool have been drinking fluoride rich water for 13 centuries and don't have any noticeable issues compared to the rest OF County Durham.

Yeah. It's not an entirely salient point. It does, however, underline the ubiquitous nature of fluorine.

The biggest source of Flourine in the environment is just the normal weathering of rocks that contain it. The biggest of the anthropogenic sources include brick production, phosphate fertiliser application and coal burning.

The minor amount added to drinking water really wouldn't be the biggest issue if it was as toxic as it's made out to be.