Deathcoil Kraken
5BB
Creature - Kraken
At beginning of each opponents' main phase, that player sacrifices all the tapped creatures they control
6/6
Now, onto the card: I don't see a mana cost for any of these and that would be pretty important to gauge their power. And the first Kraken will basically never do anything. What purpose does sacrificing tapped creatures in the precombat main phase serve? Why not make them sac them at the end step or postcombat main phase?
I don't see any major issues with the rest though some of that comes down to the mana cost. The only thing I would suggest is finding a different wording for Shipwrecker Kraken. "At the beginning of your upkeep, put X Tentacle counters on ~ where X is 4 minus the number of tentacle counters it has". Something like that. It plays a little nicer with things like Doubling Season without being too unintuitive.