Summary:
You can use tridents enchanted with riptide while standing under block. However, if you stand a little lower under the block, the trident will not work.

Steps to reproduce:

1. Enchant a trident with riptide.
2. Place and stand under a block.
3. Wait for the rain.
4. Try using a trident.
5. Go down a little lower (under the block).

Observed results:
Under blocks, the player can use the trident with riptide.

Expected results:
Trident with riptide will not work under blocks.