yep! i've used an umbrella as a way to keep rain out of the companionway and have better airflow. was very helpful on my Montgomery 15 as the boat has no opening ports or hatches (excluding the companionway).
i believe Dylan Winter, aka 'keep turning left', has given high praises to an umbrella's uses on a boat.
Sometimes just that bit of sun shade can make or break a good day afloat.
We used an Anchor Shade for years on three different sailboats (the one that has four ribs and ties down at the corners). We gifted it to a friend when we got an "official" bimini.
Even on rainy but muggy days having a bit of cover allows you to keep a hatch or companionway open. Well worth having along.
I have a golf cart umbrella that I use frequently. In the sub tropics, you quickly learn there are certain requirements and repositionable shade is a must. What I like about the golf cart umbrella is it's much larger than a typical one and most importantly, the handle telescopes and cants to about 30 degrees in any direction. It's big enough for two under it, without needing to be intimate and being able to cock it off to one side and/or lower/raise it to suit condisions, makes it a joy.