I'm late to the game here, but I'll throw in some random thoughts. After playing a Deep One themed deck, I found Lord of Y'ha-nthlei to be a great character (especially as you'll be building up a 4-resource domain quickly, leaving you with at least 1 domain with only 1 resource for awhile).
The Never Winter AP also has a new 4-cost Deep One with Toughness and Fast, which might be nice for your Khopesh of the Abyss (though personally I think that card pairs up better with a Shoggoth deck). I haven't been too impressed with Backwater Deep One or Emerging Deep One (unless your opponents regularly play Cthulhu). For a 2-cost character, I think Young Deep One might be a better option, even though he has zero skill.
Joe Sargent was also hit-and-miss for me, but I think that's because my deck doesn't have as many Deep Ones as yours. Lastly, I would swap out your 3 Dagons for 3 Cthulhu, Lord of R'lyeh. Dagon's invulnerability is nice, but he doesn't have a lot of offensive-punch. As your Ravagers from the Deep will be playing defense, I suggest taking an ancient one that can bring some heat. Anyway...
I run a combined Deep One and Serpents deck that I call my "Sea Serpents" deck. It's proving to be effective against my opponents (mostly Investigator decks, but also some Yog and Hastur) as well as my other decks (our newer players start out using my cards so I keep a number of decks on hand).
Out of the decks I play I would classify it as solidly in the second tier, if only due to its slow speed. We play a lot of multi-player games so this can actually work in the deck's favor, but in one-v-one it can go down to a rush deck if your initial card draw is poor (i.e. your starting characters are too costly). Granted, my group plays a mix of black- and white-border cards with some very nasty combos, so being a second-string deck isn't too bad considering the competition.
Here is the all white-border edition:
x3 Ocean Crawlers
x3 Degenerate Serpent Cultist
x3 Brood of Yig
x3 Lord of Y'ha-nthlei
x2 Joe Sargent
x3 Deep One Stowaway
x3 Feathered Serpent
x3 Ravager from the Deep
x3 Deep One Rising
x3 Pulled Under
x3 Get if Off!
x3 A Single Path
x3 Feeding Frenzy
x3 Deep One Assault
x3 Called by Azathoth
x2 Shadowed Reef
Cards I have since swapped out (for black-border ones) are: Joe Sargent, Yig (for a different version of Yig), A Single Path and Shadowed Reef. I found that while Joe Sargent can be effective, he took too long for me to play him, build up my Deep Ones and then have a spare 2-resource domain to drain. Killing things with Deep One Assault, Pulled Under and Feeding Frenzy just proved easier.
I took out A Single Path in favor of 3 more 1-cost Deep Ones. I used it to stall my opponents, but I figured having more characters that I could bring out during the first few rounds of play was more important. Shadowed Reef was swapped out with a card that gives all Cthulhu characters 1 terror icon (though this can be a double-edged sword).
Stand-out Deep Ones are: Ravager from the Deep, Lord of Yha-nthlei, and Hydra. Lord of Yha-nthlei is handy when combined with cards like Pulled Under, or being able to bring more characters on offense and ready a few extra Deep Ones to block. Ravager from the Deep and Hydra just make an ugly combo, especially if you have more than 1 Ravager out. Bringing multiples to the same story means you end up killing your own Ravagers, but that's a great way to wipe out characters with lots of Toughness (I'm looking at you, Dutch Courage). Then you use Hydra's ability to bring them right back again. It can be pretty frustrating for your opponent.
There are other cards I would like to experiment with in this deck, but they're in my other Cthulhu decks. Namely, I would like to try Communal Shower, Cthulhu: Lord of R'lyeh, In the Wake of the Sleeper, and Temple of R'yleh (again, a great card when combined with Hydra).