I haven't played Dredge, so I'm not the best person to review this, but I'll still try.

The Dredge content pack for Dave the Diver contains 4 new things:
1) A special type of fish called aberrant fish, which come from the game Dredge.
2) A sailing mechanic.
3) A treasure hunting mechanic.
4) Special NPC which eat sushi made with aberrant fish.

I enjoyed the content provided by this pack, especially considering that it's free. The new fish have ugly designs (in a good way) and are difficult to catch, which makes for a fun time. However, I feel like the DLC isn't fully integrated into the game: regular customers can't order aberrant sushi and vice-versa. You won't see the aberrant fish regularly; only on foggy nights and there aren't many of those.
The new treasure-hunting mechanic is just there to make money, and the new sailing mechanic (which I think also comes from Dredge) looks good but doesn't add value to the game.

The good:
The new music tracks are spooky.
The aberrant fish look awesome and fit into the Dave the Diver universe.
Easier to make money with this DLC.
It's free.

The bad:
Almost no story to accompany the DLC.
It's difficult to get into foggy nights.

Conclusion:
As a short, free DLC this was a nice addition. But there's not much to it.