The Crab King's concept is a great one, the kind of “make your own boss” type of thing, but the application was, in my opinion, not well done.
I suggest you watch Freddo Film’s latest video on the subject, he discusses well the flaws of the fight.
In short, The Crab King’s gem powers are unbalanced, meaning everyone only uses purple gems, the only way to even find it, is to explore THE ENTIRE OCEAN, which takes for ever, and if you do one single mistake in the fight, it’s game over, since he can permanently stunt-lock you by freezing you.
Here are my suggestions to deal with these problems:
1. Rebalance the gems by making them have more equal effects (except for the iridescent gem).
2. Make the gems not only buff him, but also give him different downsides/vulnerabilities (Ex: blue gem could make him vulnerable to fire, red ones could make it so he can’t permanently freeze you until your boat sinks or make him more vulnerable to freezing, purple gems could make glass cutters more effective on him or that glass cutters would lose less durability, like with shadows, etc.)
3. Make Pearl give you a map to indicate where the Crab King is when she gives you her pearl to make it doable to find him in a reasonable amount of time, especially if Klei intend to lock progress behind the Crab King, with the celestial tribute.
4. Give the Crab King a Physical attack (not AOE) that would unfreeze you so that you are not permanently frozen, like Deerclops and Klaus. This would also fix the bee cheese, and convert that to a technique instead of cheese.
Please, feel free to discuss your thoughts and ideas down in the replies.
Also, no offense Klei, love you guys 
, DST is my favorite game!