Why was controlling the sea kings such a big deal?

Some of them are really, really, really big and there are a lot of them in the ocean

Sea Kings can become incredibly huge and strong. While some characters can deal with a single Sea King relatively easily, multiple of them or even an army (there are a ton of Sea Kings in the oceans!) are a great threat to anyone, including nations, the marines or the world government.

You have to keep in mind that we're following really strong individuals (Strawhats) that meet other similarly strong individuals (Supernovae, Warlords, Yonko, Vice Admirals, Admirals); we don't get to see "small fish" (pun intended) anymore, both because the New World has far fewer of them and because the story evolved into something that simply has to show stronger characters to stay entertaining.

Sea Kings are one of the biggest threats for most people on the seas, both pirates and marines alike. That's why the Calm Belt (big parts of ocean lining the Grand Line and New World, filled with Sea Kings) is such a big deal for dividing the great oceans and the Grand Line. And the marines had to come up with special technology to have their ships safely traverse the Calm Belt.

Sea Kings alone are not that big of a threat but imagine an army of them. That’s scary no matter who you are.

Holy shit you must have had 13 days of 16 hours continuous screentime to get to the end of fishman island

You went from not watching anime at all to Fishman Island in less then 3 weeks?

Have you watched those videos of orcas coordinating to create waves to sweep a seal off of a little icy island? Now imagine an army of HUMONGOUS sea kings doing the same against any ship.

Hell, a single one of the biggest sea kings can probably just capsize anyone's ship if it attacks from below.

It's basically a super weapon. Like any strong person in the world, from Yonkos to Admirals, can defeat sea beasts. But Shirahoshi can direct them towards her enemies. It's much, much stronger than just being able to take them down. Any one strong combatant can take down any one seabeast, but what if an island doesn't have any strong combatants? Well then someone with Shirahoshi's power can destroy that island.

The only sea king Luffy dealt with ease was the one by his village which is nowhere near the size of the ones they saw in the calm belt, nor the ones Shirahoshi summoned. None of the massive sea kings have ever been shown to be harmed. And she doesn't summon just one but tons of them, like all the ones she summoned were dwarfed the main city of FMI. The ones in the calm belt were literally hundreds of times the size of Merry.

The one that Luffy punched in ep 1 is a sea monster, not a sea king.

Because of the implications.

They're in a huge ass world with mostly ocean. The anime / manga don't really show this well anymore because they skip over what would be vast stretches of water for people to travel and realistically, the show has only a few hundred that can easily take down sea kings in a universe out of at least hundreds of millions. A handful of sea kings attacking a ship is impossible to stop for 99% of the population, especially with DF users. Luffy could probably tank a few kings but if the ship is destroyed, he'd probably die trying to make it to a nearby island without plot armor.

I don't think any character could deal with multiple sea kings simultaneously (Except Shirahoshi ofc)

The sea kings in the calm belt are at least twice as big as every sear king we have ever seen.

No one can easily kill one of the bigger sea kings. Remember, it took two of the giants to just kill the island sized Sea King. Sea kings can get much bigger than that.

The ability to control an army of them can destroy any island with ease...just like the rest of the 3 weapons.

I don't think anybody has killed a sea king, or at least not the ones at fishman island, cause those were fucking massive.

The lord of the coast is extremely small, some sea kings, like the ones in the calm belt, are gigantic and we don't know exactly how many there are in there, an army of them would probably be very effective

Sea kings vary in size. Some are just big like the one luffy one shot but the largest can reach up to (and im not kidding) 5 kilometres long and shyrahoshi can control them all

Bro got to fishman island in 2 weeks in the anime 😭😭 do something else my guy

Luffy beat small ones. These ones were as big as the Noah, which is over half of Fishman Island. They are country sized. And she can get as many as she wants

Forces of nature