I've always found it strange for Marisa types, they can use their hat to float on top of water without harm but rain still will melt their hat.
water type has waterproof hat, normal type's hat doesn't have that feature, they can use it for floating, but not too long. Another explanation is power of belief: hat can't sink, but water will melt it.
Papa, you are being late...Reimu is a good girl!Stay here and wait for papa, little one
Don't run away before papa comes back for you!