Is 100 ATM water resistance realistic?

It means the watch is rated for 1000 m of static pressure, which is dive-watch territory. Treat it as suitable for swimming and diving, but have it pressure-tested after any case opening.

Does it need a battery?

No. It is an automatic, wound by wrist movement, and usually also hand-windable. Expect it to stop if left unworn for a day or two.

How does it compare with a Seiko or Orient automatic?

Those brands bring proven in-house movements, documented accuracy and service support. This watch competes on its extreme depth rating and low outlay, not on movement pedigree.

Is it too big for daily wear?

Dive watches rated this deep are usually thick and heavy. Check the case diameter and thickness before buying if you have a small wrist.

Can a jeweler service it?

A watchmaker can regulate or replace a generic automatic movement, but parts and documentation may be scarce. Factor that in before relying on it long term.