The Royal Navy retained the use of hammock well into the 20th century. However, this was only for ratings (enlisted men), not for officers. They got to sleep in proper, traditional beds (albeit with a rail to keep them from falling out in high seas), often in individual cabins. This strongly suggests that comfort was not a major reason for the choice of hammocks; if they were more comfortable than a bunk, then the officers would have used them too. The main reason to use them was that the spaces where the men slept were multi-purpose spaces. In the 20th century, the mess-decks where the men hung their hammocks were also where they ate and relaxed. In earlier times, the crew slung their hammocks on the gun-decks; as the name suggests, these were where the majority of a ship's armament was kept and operated from. Having permanent bunks would take up a lot of space in these spaces. Either the spaces would have to be much larger (which would require compromises elsewhere in the ship) or the other functions of the spaces would be heavily impaired. Hammocks could be taken down easily, to allow for more efficient uses of the space available. Hammocks were also more flexible if the ship was overcrowded, being relatively easily set up in compartments not intended for accommodation.

Being at sea over time reduces and eventually eliminates sea sickness. Any sailor that's been at their occupation for more than a few weeks probably felt no symptoms of motion sickness. The purpose of using hammocks for berthing in the 18th c was a practical one. Men could occupy large spaces while sleeping but could also easily clear those spaces and stow hammocks compactly away during other activities that required the same area. Sailors could also take their hammocks with them quite easily when they went on shore or to another command. The hammocks were woven from heavy linen or hemp duck or ticken and hemp cord and rope allowing for the beds to be constructed of much lighter and cheaper materials than less mobile wooden racks, bunks, or beds.

As to the motion, it was preferable to sleeping on a fixed bed to maintain a deeper sleep. Imagine sleeping on a fixed surface and trying not to roll off while also trying to sleep well rather than being held inside a secure cloth taco that maintains it's relation to gravity.

You will see, however, the captains' beds, those belonging to some officers, and many beds on passenger vessels to be fixed so while preferable to some, most probably preferred a fixed bed. The evidence shows that it was a practical decision for the common sailor to utilize the hammock.