Is the SRPD81K1 good for small wrists?

Yes. It is the smaller Seiko 5 Sports case, so it sits better on wrists under about 7 inches than the 42.5 mm versions. The short lug-to-lug helps it wear compact.

Seiko rates it to 100 m, which covers swimming and snorkeling. The crown is push-pull, not screw-down, so avoid diving and pressing the crown underwater.

Does it have a hacking movement?

Yes. The 4R36 stops the seconds hand when you pull the crown and also hand-winds, which the older 7S26 did not.

Seiko specifies roughly -35 to +45 seconds a day. Most owners see better than that, but expect to reset it every week or two rather than daily.

The watch on its nylon strap, a Seiko warranty card and a manual. No spare strap or tool is included.

How does it compare with the larger Seiko 5 Sports?

Same 4R36 movement and 100 m rating, but a smaller case, a black dial and a nylon strap instead of a metal bracelet. Choose it for fit and the field look; the bigger models offer more wrist presence.