No, Washi tape is water resistant rather than waterproof.
If you accidentally spill water on it or drop a roll in a puddle it will be completely fine and perfectly usable.
However, don’t try to patch a leak with it as it is not designed for this and will not work well.
Washi tape is a crafting tape which is sustainably made from Japanese shrubs such as kozo or hemp.
It is therefore paper-based, the paper is usually sealed behind clear acrylic.
If you expose it to water for a long time the paper is likely to break down and not be much use.
Interested to find out how other types of tape fare against water? Also Read: