That happened to me on our first cruise which was in January. I didn't really think about needing warm weather supplies for our cruise to the caribbean until it was too late. To make things worse, our children had outgrown their summer sandals and there was not one store around us that had sandals for sale because it was winter. Now, when we cruise in the winter months, I always buy a pair of sandals one size too large for each kid. Right now is a good time to buy sandals because most of them are on sale, and so are the swimsuits and suntan lotion.
If you can think of anything else to add to the list of summer time items that might be needed for a cruise, feel free to add to the list, I always seem to forget something.