I'd say it was the spinach, and like every one else, squeeze, squeeze, squeeeze, wring, wring, wring. You'll end up with a tiny little ball of spinach in your palm, but you can spread it back out again once it's dry. Fresh spinach will also give off a huge amount of water, so unless you blanch and wring it dry, again, I'd hesitate to use it in place of the frozen/thawed/squeezed. You can sautee either WELL dried frozen/thawed or fresh spinach before use, but in an omlette or a pasta stuffing, I'd be afraid it'd leach too much moisture even after a good stiff sautee.

Seriously....squeeze until you think it can't be squeezed any more, then squeeze another 3 or 4 times. A towel helps (cloth, the paper towels will just integrate into the spinach), but just your hand is fine.

I make egg white omelets with frozen spinach at least once a month. The key is to wring the hell out of the thawed spinach. I just use an old kitchen towel and then take out the day's stress on the poor veg. But the result is a perfectly luscious egg dish.

Were the eggs watery or the spinach? It's possible your eggs were not at their optimal freshness. The quality of your egg may have been the issue. Not to say you have poor quality eggs, but if they were regular supermarket eggs, it possible that they were not AS fresh as a farmstand egg, and there for have watery whites.

If its the spinach, try folding fresh baby spinach into your omelet the heat from the eggs should wilt the spinach

I Sauternes the spinach gently with aromatics. I consentrate flavors and dry out the veggies that way. Its worth the few minutes it takes.

Even fresh spinach contains a lot of water (which is why a big bag cooks down to such a small amount) - but frozen spinach contains an almost insane amount.

Only solution I've found is very low tech - just defrosting and draining it in a colander and then squeezing it (with hands/paper towel/in cheese cloth/whichever) more times than could possibly seem necessary. And then squeezing it some more. And more...

When I use frozen/thawed spinach in baked pasta dishes, I squeeze handfuls of the spinach to wring out the water. Not crazy about losing what could be vitamins down the drain, though.