We've done RCCL cruises from Barcelona several times. Easy Jet is usually the cheapest if booked far enough in advance. We always go the night before and book our own hotel, also through easy-jet. Most inclusive booking agents fly the night before but we are still in pocket doing it ourselves. The only problem would come if things went badly wrong and you missed the ship, then the cruise side of your booking wouldn't bale you out.
Do the maths and if it works out ok then it is ok but remember that all transfers costs to and from the ship must be added on and if you are delayed and miss the departure they will not wait for you. We have benefitted from cruise only purchases but there are things to beware of. Neil Dow.
On paper it is easy to do - remember if you book with an airline like EasyJet and they cancel that flight they are not likely to have as many flights to get you there. If you miss the ship then it is your own responsibility to make your own way to the next port to catch her up.
Most times this is what we do. The only exception would be cruises starting/ending in different cities/countries. We always fly in the day before in case of any delays. Taxis are usually cheaper, quicker,less stressful than cruise transfers. PLUS you have control of your own schedule - a big point for a control freak like me LO.
The last answer is one to consider very carefully. My advice would be to stay a night in port before the cruise, just in case. It's false economy to save on the flight simply to find the ship has sailed without you! How reliable is the flight company? We've flown with Easyjet & they are great, but you hear of some horror stories with flights just cancelled & no replacement. At least a day earlier than the embarkation would be a peaceful stress-free buffe.