Shell Shield is placed on the ground instead of channeled around you and lasts 2s longer.
Increase the Health of Shell Shield by 200-1000.
Reduce the Cooldown of Shell Shield by 1-5s for every 2500 damage Absorbed.
Reduce the Cooldown of Dredge Anchor by 0.5-2.5s.
Increase the duration of Shell Shield by 0.6-3s.
Regenerate 1 Ammo every 0.5s for 0.5-2.5s after activating Dredge Anchor.
I'm kind of new to this whole guide thing, but I've had quite a bit of experience playing koa for awhile now. This is a loadout that I prefer use whenever I'm able to offtank and want to put pressure on the enemy backline, but still maintain a certain spacing between my team and the enemy team. For a brief overview of the cards:
Carapace V: necessary in any half shell build combined with Barrier Reef because otherwise you won't be able to get the benefit of maximizing the CDR on your shield.
Barrier Reef IV: I run it at least at a IV for this build so I can have my shield up more often to keep on pushing, but it can optionally be brought up to a V if you want to get your shield out more like most standard half shell builds.
Strong Arm IV: helps to get your hooks out more often and after the current Radiant Stars patch you'll be able to have your hook CD at 11.6 seconds before chronos. You may also run it at a V if you like to maximize the hook CDR, which if the case, I would recommend pulling Barrier Reef down to a III.
Ancient Resolve I: here purely just as a filler card. You can run anything else if you want; other suggestions could include Surf, Determination, or Tidal Grace.
Salvage I: personally, I feel that Salvage is the best filler card for koa having it means that you'll have that extra shot every time after throwing out your hook.
For as far as items go in matches, it really depends on the map, enemy team comp., and your team's comp. Although, a standard approach I usually take with this build is:
Cauterize: since you're an aggro tank with this build you want to ensure that the enemy Vik your after doesn't suddenly get burst healed for almost full health from his Corvus.
Rejuv.: PSA: help your healers!
Blast Shields or Haven: depending on enemy team comp.; purchase based on your own discretion.
Chronos: helps to get your shield up more often especially if you don't get the double Barrier Reef proc.
Nimble: I rarely if ever purchase this item when playing with this build, but if you need that extra little boost to get your fat turtle legs to chase down that enemy healer, then go for it.