The weather here in Barcelona is most often very good for sailing and boating. But what time of day you go can make a difference to the seas and winds. Firstly I must say that these are very much generalizations and the weather is different everyday. However the mornings are most often calmest. If you are considering a stop at anchor for swimming then it is really worth going early. The seas must be very flat with low winds for the boat to stop and for it to be fun to dive in. If there are regional bits of weather passing through then the winds are usually low in the mornings and blow from the north. The best places to anchor for swimming are more protected from light morning winds from the north. Then in the late morning or early afternoon the winds switch to coming from the south and gradually increase until they are strongest in mid and late afternoon. If it is a very windy day this can make the sea choppy mid to late afternoon. So you are taking a bigger risk that the weather won't be perfect. Winds tend to increase more strongly in spring and early summer afternoons because of the difference in temperatures between the city and the water. The boat can be out at sea until the end of sunset so a sunset cruise can also be a good time to be out. The winds usually drop off quickly in the late afternoon calming any chop. The hour before sunset is usually the best.