Killing them with a trident with Loyalty is the best way, but of course you need to kill a lot of them before getting enough tridents to repair one to good condition unless you can put Mending on it.
If you have good armor, preferably with Projectile Protection and Depth Strider, you can just swim up to most trident wielders and slash them with a sword, some however are very good at evading, and with less good armor your health can quickly be worn down. Even with pretty good armor, facing two or three trident wielders can be a serious problem.
If you can lure them to the surface you can shoot them with a bow, works best at night, standing on the shore with something to stand behind (or just far enough from the edge) so they have to get close to hit you.
It generally takes me 1-2 Shields (depending on how many Hostile Mobs there are, how slow I am - there's Knockback from the attack - and my luck in Moving-correctly, in Water) for a Drowned, pretty-much my highest Shield consumption rate. This is if you're also-trying to use a Looting-Enchanted Sword on them, to increase the drop rate of the possible-trident(s they're throwing at oneself!).
Yeah, any-more than 1 trident-wielding Drowned is a Problem, on Hard Mode. I'd start to use a Bow, too (to, you know: " .. 'Survive' !" ; as goes the Main Menu, Splash).
I am pretty sure that a Shield offers less-overall consumption-of materials than just Armor-alone (plus Enchantments would consume-more, Exp. Pt's. , for this of course, less-tangible resources). That said, almost makes me wish for a: "Shield 'of Infinity'," heh; can't remember if they're Enchantable or not.
With my sword having knockback, I tend to use the tried and tested - block (With shields), swim up to drowned, swipe with sword (Sometimes having to block on the way) swipe... and repeat. Always works well for me.
When approaching them from a distance I find that spiraling in towards them or moving up and down, like a roller coaster, can cause them to miss most of their throws, once you get close they'll hit you anyway but as long as the trident still takes a while to get to where you were when it was thrown it's nice to no longer be in its path.