Wow, you're so quick at these things!
Yes, that sounds like a really good Diver to me. Sure, it could be played the other way around, removing the Survey stuff from Diver and instead adding a requirement to the underwater Survey table that most things requires Diver to explore - but it makes much more sense to me as you've written it. I think you have nailed the Diver talent pretty spot on. It's powerful but also very situational.
The Survey table for underwater - good stuff! I've added my input/reflections to each line.
- Abundant Fish: Bonus when fishing. -- Yes!
- Purple shells/Oyster beds: Dive for food/pearls. -- yes! I like the combination of food with a chance of a pearl.
- Huntable sea life: Allows hunting. In a lake maybe: otters, beavers, ducks, geese, muskrats, and swans? -- Yes! Though, do you mean as in Abundant? (bonus to hunting?). I could totally see that these made it into a regional based hunting-list (also somewhere down on my own todo-list...)
In an sea/ocean tile maybe seals, sea turtles, dolphins, sharks, whales, and sea birds? -- Yes! It's cool if Diver allowed the character to hunt for food/meat othwerwise not available.
- Kelp forest: Vegetable resource, maybe fertilizer, could maybe work like some kind of cloth too? At a minimum it could be used for ropes and so. -- Yes! Unless you're complicating your crafting tables too much... (I can't recall if you have something similar in there)
- Corals/Seashells: Crafting material? Construction? Jewelry? -- Yes, atleast a straight off "2d6 s" value type of reward? But you are the crafting master, it's cool of course if it can be crafted into a bonus to manipulate.
- Underwater caves? But what do they add? Minable resources. Feels too fantasy? -- Makes sense as well! I mean, there's a chance of ore everywhere... And dangerous to mine.
- Sunken Treasure: Could be an old ship or so? -- Yes! Classic. Could be presented as trade goods, with some of it unspoiled.
- Sunken/Underwater settlement: An old settlement that once where on land, that you could dive for simple finds in? -- Yes! Dangerous, but sometimes fruitful. Additionaly; Maybe a chance of being a wizards tower kind of thing.
Or maybe an active settlement owned by amphibians or other water dwelling creatures. -- Yes! But the character has spotted it first, from a safe distance.
- Geothermal Vents: Probably a no on this one, or maybe in the Bloodmarch? But what do they yield? Rare minerals perhaps. --- Mmm, it sounds right for Bloodmarch.
- Some super rare phenomena, that lets you travel through a vortex to another random water hex? -- Haha, yes, why not?

(Perhaps with the equivalent of on underwater Portal guardian)
- Some wood resource that somehow got improved from being submerged for 100s of years? Or probably not? -- Yes, maybe not submerged for 100 of years, but an underwater living tree-ish plant that has the properties of one of the stronger wood types? "Giant kelp forests" kind of thing.
Additional random ideas:
- The bones of an undewater giant beast (the equivalent of the random encounter "28. The Bridge" from BoB).
- Random underwater monster from the tables. But the character has spotted it first, so it's safe to just swim back (and gather your crew of fellow Divers to hunt it

. (The claw marks of a water troll, a hunting amphibian, a sleeping giant octopus...)
- X units of Luminescent algea, counts and weights like torches.
- Statues and other remniscents of ages past, rewarding the character with legends.
- Underwater elemental springs ... Hints of weird underwater vegetation and water of a type not found elsewhere
- Clay... Both Pelagia and Burning Pits of Llao-Yutuys has a focus on evil clay makers, my players has noted. There should be a bunch of sources of good clay around.
- Herbs... of the interesting kind.
- Encouner 25 Store orm from BoB... and 36, the cast-away.
- Time-weathered journal...
- Sunken shrine to the goddess Flow, with a boon and bane to a worshipper. (I have forgotten if the books have something on her, except for the water in The Hollows). Or the goddess Klay(?), as orcs worship her.
- Old battlefield. Maybe Sauri and Amphibians had a history of animosity...?
Btw,
Doing this in The Bitter Reach adds some obviousl dangers of freezing to death. That's something I wanted to pitch to you around as well. Instead of flat out dying when broken in Str or Int while Cold, I started to use your Critical Injury: Burn table. Just slightly modifiying it to fit frost/freeze damage. It has worked very well, especially in combination with your Heat rules.
Btw 2, here's my pitch on Survey:
Remove the Survey stuff from Builder. Make Survey a standalone Talent, that only is available if you choose the Survey module. This would sort the module overlap.
In the Survey talent, already at rank 1 or 2 mayhaps, you can choose to add +10 or -10 to the Survey table roll. I mean, in the sense that rolling 01 means 11, and 71 means 61.
On 61-66 there are resources of the land, the stuff you want to craft armour, make weapons, and build your stronghold from. Maybe also things like herbs that grow here, maybe food sources.
On 11-16 there are another type of findings, the things that explorers/adventurers out in the wild (and that would likely never return to this hex) would be more interested in.
Quagmire and Mishaps - totally agree that it's fun when different terrain types have different amount of risk involved. But, that could be achieved through the Mishaps table, could it not?