Friends of my parents did transatlantic on celebrity. During the crossing they said it was upper 50s to low 60s, not factoring the wind chill.
As for the ports, We went in the beginning of May, with temps in the high 60s and mid 70s. We experienced a thunderstorm in Spain on our arrival, but sunny and cool the next day. And we had light showers in France. Every port started out cloudy, cool, and breezy, but by mid morning it warmed up. My best advice would be to layer since the weather is so unpredictable. The friends each brought a light jacket and windbreaker, while we each brought a light jacket and sweater for layering. Since we flew from SFO, it wasn’t a big deal because we were already wearing them. Neither of us brought umbrellas since Celebrity provided them. Definitely make sure you check the weather before you leave.