In real life, sea cucumbers don't mate, but reproduce by throwing out large amounts of eggs or sperm into the water. Also are sea cucumbers asexually.
this means that if you have one sea cucumber, more will appear over time as long as the'res enough living ground.
ingame this means you can put one in your pond and slowly get more. There is however a limit to how many you can get. Either there's no more space (1-2 bloccks of water per cucumber) or there's just too many. Like 15 in a 100 blocks radius and they stop spawning.
This way you can get one cucumber and not have to go back and forth to the ocean again and again to get more seacucumbers, because that would be almost the same thing as going back and fourth to the swamp for clay, difference being that at least sea cucumbers are renewable.