Hi, You should be able to do a cheap "air-test" in any bigger shopping mall for 5 euros. If it passes the 30m test it will be good for more, essentially for light swimming in a pool or the sea.
Things needed:
1. check that You buy a waterproof rated diving watch instead of a chronograph or dress-watch (TD-rating of 50m or 100m, not less), these will be few
2. the crown o-ring and backcase gasket should be silicon-greased by Yourself and afterwards well tightened (they normally come very loose on all of them)
3. in 90% of the cases these 2 simple things will do