I made a book last night entirely built around clerics! The new clerics from this set are amazing! DB initiate might be my new favorite lvl 1 critter
you might want to hold back on that one
I just played my first game as the Siren, against a Malakai Priest, and I might have something to say about clerics.
First off, to answer OP, what I learned throughout my game is that there are three essential creatures for the Siren: Kraken, Shoaldeep Crocodile (one-two is fine) and the Sherean Leviathan. These creatures can pretty much help you dominate a game with the Shallow Sea, and their high dice rolls are essential to victory. Grapple also playing a huge part, and I think that the Kraken has to be the biggest thorn on my opponent's side throughout the entire game till the end, with the 6 dice melee attack being a very nice attack to have.
Now, about clerics, they are generally weak in life and attack, something I am guessing most of the players here know. While Dawn Breaker's Initiate helps move crucial creature around, they were easy meals for my Crocodile. If they are not well played properly, clerics are creatures that the enemy can easily ignore as it doesn't have any dangerous effect on them. I implore for you to not make the decision of a massive cleric book, as you'll end up regretting how weak your army is.