At sea, the ship will steer around isolated showers, if possible. In port, showers are often sporadic (in my experience), and many times if it is raining in the morning, the sun will be out in the afternoon. Of course, there are days when it will do nothing but rain, but that can happen anywhere. The combination of heat and humidity often results in scattered showers.
I've had cruises where the weather in the ports called for showers and have seen no rain at all. Hope you have the same luck!