I'd think that lithium toxicity would be enough of a danger that it wouldn't work out. People taking lithium as a mood stabilizer need routine blood testing to ensure safe levels of the drug are in their body.

Therapeutic doses of lithium are almost certainly thousands or millions of times higher than what's found in sea water.

It’s a compelling idea, but the science is spotty. There was a popular article in the nytimes suggesting this a few years ago that got people talking, and here’s a good critical response:

http://www.rff.org/blog/2014/should-we-all-take-bit-lithium

You can buy low-dose lithium orotate supplements very inexpensively in the US however, if you want to do an n=1 experiment.

7-Up started as a lithiated soda. The atomic mass of lithium is just under 7.

Yup. It's interesting how many modern drinks came from medicinal origins. Original formula Coca-Cola, of course, famously had a healthy dose of cocaine. Tonic water still has a bit of quinine in it, but originally it had much more, and was consumed as prophylaxis against malaria. Then there were various "bitters" and "digestifs" that had purported health benefits. Jägermeister has a whole witches brew of herbs and roots in it. And so on.