We usually take an umbrella. The reason is: Each time we've taken one we haven't needed it and the one or two times we didn't we sure could have used it.
A small, compact umbrella is a good thing to throw in the suitcase. Ponchos are even better. They take little space and have almost no weight. On most cruise lines they will sell ponchos, at the gangway, when it is raining in the port. I think the cost is somewhere between $3 and $5. You can buy them at a sporting goods store for .99 cents.