We have always used cash. You have to carry it as far as the ship. Put it in the safe, and take out as you need it. With Travelers checks, you have to find someone to cash them. If you get the ship to do it, sometimes long lines.
also, are there easily accessible atm's in ports? if the ship's atm runs out of cash? I was going to use my mastercard/debit card and bring along one credit card for emergency's sake. I only plan to bring 50 $1 bills for baggage handling tips etc. would that be enough for a 4-night cruise?