Seawater contains Deuterium (D) which can be used in a full cycle fusion reactor. Basically D+D creates either Tritium (T) + H or He3+ a neutron. Those products can then react with one another. The easiest reaction to do is D+T since it requires the lowest temperautures but Tritium needs to be manufactured which can be difficult.

You can find it in this table on Wikipedia: https://en.m.wikipedia.org/wiki/Energy_density_Extended_Reference_Table Or at this site: https://epress.lib.uts.edu.au/student-journals/index.php/PAMR/article/download/1383/1464?inline=1#:~:text=Energy%20density%20of%20Deuterium%2Dtritium,reactants%20and%20products%20%5B1%5D.

The key detail is that pure uranium is only about 1/3 the energy density of Tritium-Deuterium but fission rods are only a couple percent U-235.

This isn't really that accurate. Yes, both fission and fusion have zero carbon footprint and in the end will (at least in most cases) just boil water. Fusion however, if fully realized in its most ideal form, would allow us essentially turn seawater into fuel (so the source is basically limitless). Also fuel for fusion contains abot 100 times the energy per pound compared to fission. Finally, fusion has much less risk (but not zero risk) of danger from radioactive byproducts.