<<1) What are the weather conditions likely to be-ie will it be rough water?>>
It all depends on the current weather in the Atlantic. April is one of the quieter months, but one never knows. The Legend of the Seas is quite seaworthy, so I wouldn't worry too much about the ocean condirions.
<<2) I was on Brilliance on the Med Cruise out of Barcelona Last October- it was great. How will the Legend compare?>>
The Legend is an older ship, albeit still in great shape. The difference between the Brilliance and the Legend is considerable, but both are excallent ships.
<<3) I am travelling single in a cabin which shows extra sleeping available - does that detract from the comfort of the cabin. Wont be using the additional bed(s) of course.>>
I always get them to put two singles together for more sleeping room. If they can't then I use the extra bed for more storage of my stuff.
<<4) Ant other tips info appreciated including what the earliest boarding time will liekly be.>
Depending on when the ship arrives and departs the port, I would try to board 3 to 4 hours after arrival. If the ship arrives at 8 am, try boarding around noon, or slighjtly before. Royal Caribbean is very good about boarding people the second that they can and I have always had luck getting the the terminal early.
Have a great cruise!