The best advice I can give you is to be prepared for anything. If your cabin is on the starboard side you will get the best sun in the afternoons but it can still be cold or windy so I would suggest bringing windbreakers and possibly something fleecy.
The weather can be very changeable but you can get lucky and sit out in a bathing suit too.

But remember the ship is always air conditioned. So in the show room and dining rooms it can be drafty even when its warm outside.

I just did a tropical cruise and it was hot and humid outside but for the shows i always needed a shawl esp for certain sections of the theatre.

Ahotpoet is right....layer and be prepared for anything!

April can be still cold in the morning, at night and windy especially getting close to Denmark and inside the ship with air conditioning. I would bring any kind of cloths: from a heavy jacket to a bathing suit.

I went on a TA last year from Miami to Barcelona. I weather during the crossing to be warm more often than not. If you bring layers making sure that that you have a warm sweater and wrap along with an outer jacket you should be fine. I assuming you're taking NCL and I don't usually find their smaller ships to be chilly and I get cold quickly.