How far does an arrow travel underwater?
I did a test in a Creative world, and I found out that:
- Crossbows and bows shoot about the same distance from the player underwater
- Crossbows and bows shoot 12 blocks away from the player underwater
As for fireworks, they seem to work the same way as they do on land. They aren't very good weapons, because they can only be used on a block, and they go pretty high. This means that you are most likely going to be able to miss your target.
Conclusion: Bows and Crossbows shoot 12 blocks, fireworks work normally, and none of these are very good weapons to use underwater.