Weird, been a while since I've played but I thought spear fishing could only be used from the back half to your team's back half while seaward slash could only be used from the front half to your team's front half.
I'm gonna do some 'covesplaining here so don't get too upset folks. It's really not as terrible as some people make it out to be. There's definitely a difficulty spike no doubt about it, but there are measures you can take to make it a lot less daunting.
First if all of your cove expeditions have failed try bringing a man-at-arms. Even if you stick him in the third row back. While the cove does have some high damage attacks they're all fairly predictable. Like I said earlier (and unless they changed it) seaward slash and spear fishing can only target one of two people depending on where the pelagic grouper is standing. Man-at-arms can usually intercept their attacks. Also, octocestus can only hit your front half, very predictable and easily blocked. Hell, if man at arms is in your front row you can intercept octocestus 100% of the time and just cure the bleeding. You can use rampart to stun something and get your man into position at the same time if he's in the third row.
Second, pelagic groupers and shamans are very fast and somewhat fragile. If you've got your team loaded down with -speed trinkets you're going to take an ass kicking at the start of every turn. Instead, take off those trinkets and put on any +speed ones you might have found (if they exist?). Bring bolster with your man-at-arms. If you get turns in before the groupers you can completely negate their damage for that turn by 1-2 shotting them.
Blight IS the better option for cove, bringing a plague doctor usually works out for multiple reasons.
On difficulties above apprentice you'll start running into giant crustaceans. They use arterial pinch which causes massive bleeding, the plague doctor can selectively remove that from anyone. Or spend her turn blighting the giant armored crab. Or just blighting/stunning every damned fish in the cove. She's really useful here.
Bleed resistance on fishmen just means not bringing a bunch of bleed abilities. "If it bleeds" on the barbarian is still good because of its range. Bleed is just a bonus and it still gets through despite the resist.
"So what you're saying is in order to clear the cove I HAVE to have a man-at-arms and a plague doctor?" Nope. Those two classes just shine there and do well with a lot of team comps. If you're struggling, try them out, bring useful abilities, get rid of your -speed shit so you have a chance of taking initiative over groupers and bring stuns. I believe stunning a pelagic guardian will bring down his guard on his ally. Crusader and barbarian are both capable of this, not just man-at-arms. You don't need dots to kill a guardian efficiently, they're meant to be beefy as hell and will take multiple turns to kill no matter what.
An upgraded occultist does well here with his barrage ability. He also pairs well with the plague doctor because of her ability to fix botched wyrd reconstructions if it's necessary. The arbalest is great because of her group speed buff from camping. She can also pick off shamans, prevent disasters with rallying flare (3 monsters in the cove stun regularly), and her bandage can be good with wyrd reconstruction but it takes up turns.
Finally just a general tip for people struggling: if you have stuns/intercepts and the only enemy alive is weak, predictable, or susceptible to stuns, drag out the fight to stitch your dying up. I know that the developers programmed a "countermeasure" to this scummy tactic, but it's really not that effective. If your team is somewhat relaxed yet almost dead you should be taking advantage of this.
But yeah fuck drowned thralls right in their bloated torso holes. That's the official strategy guide answer for those bastards.