While being on a server usually means low spawn rates, skeletons, creepers, zombies, spiders etc spawn commonly on a nearby forest biome. I've seen hundreds in the past 3 - 5 days. However, unlike you'd expect for someone who lives in an ocean, I've only seen 4 naturally spawned drowned. This makes it practically impossible to get a trident.

Even in SP, in a buffet world of deep lukewarm ocean, on hard mode, I only find a small few spawn.

Waiting about five mins starting at 14000 ticks into the day/night cycle and first killing all entities, I got 7 drowned.

Waiting about 5 mins on a small stone platform I placed above the ocean after the drowned test, and again 14000 ticks and killing entities first, I got 12 zombies, 8 creepers, 6 spiders and 11 skeletons.

Pictures attached.