As a long time OM player just returning to sh4 for 2021, I have been testing/playing Dark Waters (DW) quite a few hours for the last 10 days or so. Dark Waters is strongly recommended.
lurker did an amazing job by creating OM <- truly an amazing one-man giant team here!
DW team standing on the shoulder of the giant, further improved the experience not only fixing bugs but overall improving the experience with additions, tweaks.
I am not affiliated in any way but as a lover of Uboat experience genuinely excited about how good OM+DW got over the years. Some of the DW improvements include more people/activity in Uboats, Uboat lighting improvements, type-identifiable ship smoke etc. (edit: frequently (every time?) deployed lifeboats!). Further details partially listed in 1st post in DW release thread. Even more information scattered across many dev posts in that thread.
Without saying anything bad about OM, I definitely encourage you to try out DW. In 2021 no need to play only the base OM as you can get OM + fixes + improvements + developer support for the same price
In the name of total transparency the only issue I encountered was "radio not working" which very well could be down to an issue on my end as I cannot see any others posting about this issue. It was an easy fix anyway (posted in DW thread in case you experience the same). Weirdly even this 'issue' was a good experience as I posted a question about it and one of the devs replied I think within the same hour (on a side note: I wish at work our commercial vendors replied that fast!).
Thanks to lurker, fifi & entire DW team!