The insulation on the tank will dry out- if it was fiberglass it might not recover it's initial R-value but a retrofit tank blanket should keep it's efficiency up. If its was foam, it never lost it's R-value.
As long as it remains dry corrosion won't progress. If any electrical connections were soaked there could be issues down the line, but that would likely be easily repairable stuff.
You may see some corrosion or bubbles/flaking paint on the exterior jacket from moisture that got trapped on the interior side, but that would have little effect on function, only appearance. (And if you covered the thing in retrofit insulation you wouldn't have to look at it anyway.)
I'd wait for (but not expect for) it to fail before doing anything beyond inspecting & cleaning any electrical connections below the high-water mark. (Did it reach as high as the lower heating element?)