You can only throw Riptide tridents when touching water

You need to be touching water to use riptide.

I want to add to this bug report to prevent confusion of any new people that come to this looking at it. There is a weird visual inconsistency with rain sometimes where you will see it raining, but you won't see the water droplet splashes on the ground. If this is the case, it is visually raining in the location, but not truely considered raining, and due to this a Riptide Trident will not work.

This tends to occur when a biome where it rains is next to a biome where it doesn't rain where there is the biome's edges if you will. I'm not sure if this 'false' visual only rain at the edges of said biomes is intended or a bug, but that is what it is and is the reason why along those edges the Riptide Trident will not work despite seeing rain. So make sure if it looks like it is raining to look at the ground and check to make sure water droplets are infact coming off of the ground to ensure that the game considers it to be truely raining in the given area you wish to use your Trident and not just a visual effect at the edge of a warm biome and non-warm biome.

I can vouch for what I stated here on at least Bedrock Edition. Java Edition would need to be verified.