A cruise only is where you must get to the port od departure yourself. Miss the flight or it is delayed then you are on your own to get to the ship, wherever it might have sailed to!
Fly cruise then the operator will look after you if there are flight problems. I have heard that sailings have been delayed in such cases.
Fly cruise price covers the price of the connecting flight,cruise only is without the flight you have to arrange and pay for the flight yourself this is maybe you would like to spend a few days either end of your cruise in the embarkment port.
Cruise only means no flights included.
Fly-cruise means you pick up the ship at a point in it's journey and you fly from your home country to that point e.g. World cruise but you ar doing a far east cruise you will need to fly to Singapore or Sydney to embark. The flights to and from will eb included in teh price. Cruise only means you pick up the ship at a port and have to make your own way there at your own cost.
Fly cruise is where cost of flight is included, cruise only need to make own way to por.
Fly to your port of pick up, then cruise to your destination, possibly sail back to U.