Yeah, it sounds pretty silly, doesn't it? For metals, if it's saltwater the artifact has been enduring, often a very thick layer of corrosion has built up. That layer is permeable, so soaking it in water ( hopefully distilled water, without other dissolved minerals) leaches out the salt- that means soaking, changing the water for new, soaking again. Basically, washing the artifact.

For iron objects, there's a coating of rust. Regular rust (Fe2O3) is active- it keeps corroding the base metal. A very common treatment for that does involve immersing in a tank of a somewhat dilute electrolyte, sometimes water and sodium hydroxide ( lye) sometimes water and sodium carbonate ( which is more docile to handle than lye). An anode is wired up and put into the tank ( platinum is great but much less expensive things will also work) , the artifact is wired up as a cathode, and a low voltage DC current is applied. Hydrogen atoms in the water are attracted to the negative cathode, forming at the surface of the iron, and combine with the oxygen in the rust and reduce it from iron oxide to iron...the bubbles formed can also knock off some crud and loose rust. It can be a very slow process, which is maybe why no conservator I've met seems to be in a hurry....

That's the very bare bones of it. There are other techniques that can be used as well (not everything can be put into a tank of electrolyte) and conservators will employ a variety of them. Wooden objects also will often be soaked to remove minerals, treated to kill bugs and fungi, and then water-soluble resins then worked into the wood to keep it from shrinking to bits. If you get to Newport News VA, the Mariner's Museum has been conserving the turret of the Monitor for years , and you can see some interesting info on that here. Texas A&M posted a manual for conservation of underwater objects here, if you'd like to get even more detail.