I've been hit by this a few times, but I'm so used to hitting the equip button that it's just second nature for me now. Any time I'm not swinging a weapon or blocking I put them away to save stamina, so, unless the water is a surprise, my weapon and shield are not going to be equiped anyway.

First sign of danger, I also ensure that I have the right weapon and shield equiped, and then put it away so that I can quickly re-equip it before I engage.