Bug Doctor

I looked into this in 1.21.11 (unobfuscated) and found a likely cause in LivingEntity.travelInWater.

It seems that the normal water friction (slowDown, usually 0.9F) is still applied while isAutoSpinningAttack() is active. Because of that, Riptide momentum gets dampened almost immediately, especially when combined with Depth Strider or high WATER_MOVEMENT_EFFICIENCY, which makes the player feel like they stall in water.

A possible fix would be to disable water friction during the Riptide attack, e.g.:

float slowDown = this.isSprinting() ? 0.9F : this.getWaterSlowDown();

if (this.isAutoSpinningAttack()) {
    slowDown = 1.0F;
}

This would preserve the intended Riptide momentum in water.