Short

With short passages under (floating) water, arrows may or may not make a splash sound depending on the distance from which the shot was made. Longer passages will always make the splash sound. In all cases, the arrow does not visibly touch the floating water block.

Longer

Test setup:

Have pool of water (source blocks) above a "floor" made of bottom slabs or upright stairs blocks. This leaves a visual air gap (of half a block height) between the water blocks and the slabs/stairs, either wide flat open gap with bottom slabs, or narrow tunnels with stairs.

Or download the attached world save with a suitable test setup and instructions.

Test action:

Shoot arrows horizontally through those gaps, so that the arrow does not hit the slabs/stairs or the water blocks.

Current results:

If the player is at certain distances and the length of the gap (in shooting direction) is short enough, there is no splash sound.

But if the player moves to another distance (e.g. about 1 block further away), or the gap is long enough, there will be splash sound(s), even if the arrow doesn't touch water.

Expected results:

One would expect water to flow down to fill the free space in the block/stair block's space, and thus arrows to make sounds. However, at least visually the water does not flow down, and thus shooting arrows through should not make sounds.

Or, very least, the sounds should or should not be played consistently, not depend on how long "air gap tunnel" to fly through, and especially not depend on how far away the player was from the gap.

Perhaps...

... The splash sound could be triggered by a larger hitbox of the arrow touching the water above, but the hitbox collision is checked only at specific intervals, and during its flight, a fast arrow might be at first just before the water, and at the next step already passed the water, thus not triggering the splash sound. With a longer gap/passage, those hitbox checks would always "hit" some of the water blocks above the path.

(As of 22w17a, attached a world save with test setup. A superflat with "test range" and instructions.)