Horolotard

Tissot still makes pocket watches that almost fit your requirements, some models for example being skeletonized with display caseback and 100m water resistance but its not under 400 dollars

You have to just wait for new listings sometimes, but an underrated brand favorite of mine is Sector watches from the 90's. New old stock of good ones pop up from time to time, especially coming from Italy so it's worth searching for them in French too lol. The watch I wear the most lately is my Sector 450 diver watch I sniped someone in an Ebay auction for and got for $32 with tax and shipping. This is the same watch listed for $130 and honestly even at that price it's a good deal. The sapphire crystal is very thick and the watch itself is only 8mm thick, 7mm excluding the caseback. The bracelet is top notch and like most 90's Sectors has a diver's extension clasp with microadjustments. You never see it but the movement is actually pretty for a quartz too, with vibrant blue circuitry and stainless steel construction.

A brief(ish) history on Sector and why these tend to go under the radar and sell for below their worth. It was founded in 1973 by Filippo Giardiello in Italy, and they were most popular in the 80's and 90's filling the niche of sporty and masculine diver watches. They were primarily manufactured in Switzerland with Swiss movements and branded themselves as Swiss watches, around Y2K they went under and in 2001 sold to BVLGARI. Sector operated under BVLGARI until 2006 when it was sold to The Morellato Group, an Italian jewelry company that had no experience with the watch industry that was looking to expand their business and try to pull a Cartier... in the early days they did release some decent albiet lower quality watches, but they absolutely tanked the brand into obscurity with boring, unoriginal, uninspired watches from questionable origin and they did it right as smart phones were coming out and social media like Reddit was beginning to exist

Hit or miss but yeah you can absolutely find some really great deals at auctions and estate sales

This is the only legitimate brand for a true military watch. As per Wikipedia:

Marathon started supplying watches to the Allies of World War II in 1941. Today Marathon manufactures watches that conform to United States Military Standard MIL-PRF-46374G, as well as those of other nations. Marathon is the sole supplier of watches to the United States Armed Forces.

Gravity does have a small effect but the exact effect is not universal, meaning different watches will slightly gain/lose seconds differently stored in the same position

Most people don't actually ever go further than 5 meters deep with their watch, but a modern watch rated only 5m has been made with absurdly low specs, and AFAIK unless we're talking about a certified divers watch, then specs are more of a 'manufacturer's suggestion' than an actually proven depth rating. So a modern watch rated 5m, when they easily could have slapped 50m on it... even if 5m is the deepest you'd actually go its basically the manufacturer suggesting to keep it away from water. Pointless and confusing but that's the way it is

The best deals around are Ebay auctions on old quartz watches, like G-Shocks. If you wait to place your bid and snipe you can get a crazy good deal. Ebay is also a cesspit for fake Seikos known as frankenwatches or Mumbai specials though, never buy from India and I'd avoid older Seikos and other vintage watches in general on Ebay without knowing what to avoid

Seagull movements can make beautiful watches, I just have a hard time with the idea that being what it is, a true Chinese chronograph for under $300.. The day it inevitably needs a service, It's basically been totalled. Even buying a new movement and swapping it yourself is high risk and low reward compared to.just replacing it