I was thinking about how water tankers on land have to be full in order to travel successfully

Because a water tanker or even anything that hauls anything liquid usually has to be full when traveling. You don't want it to move around and tip the whole thing over. So, you usually have to fill a semi water hauling trailer completely full with water or whatever you are filling it with so you don't have an accident when all that weight moves around. If there is little or no air in it then it stays more stationary. It might be heavy but it doesn't turn over going around a corner from water or whatever sloshing around. So, Flame's Land Suit or ATV of the air, land and sea and space would likely be like this where there are no air pockets to disrupt it's movements over land, sea or air or space.

So, just like water tankers or fluid tankers have to either be full or empty to successfully travel anywhere without accidents. Flame's society of Octopi on his planet would have to do this too with their spaceships and land and water and air ATVs. However, water is relatively heavy so taking off into air or space likely might be difficult weight wise unless they have solved this problem somehow of all the extra weight.

However, scientifically, all this could be accomplished one way or another. For example, one way you could solve this engineering problem would be to do something like we do with Geosynchronous satellites. You could have a hose that was spinning with the planet and allow the vacuum of space to suck up the water like a syphon so it wouldn't take any energy to move the water up the flexible pipe. First you might establish the Geosynchronous satellite or hose in space on a not very windy day on earth or up to 200,000 or 300,000 feet in altitude. Then you have the hose out far enough in space where the weight of the hose is counterbalanced by the centrifugal force of the spinning planet. Then you open the end of the hose and allow the space suction of no air to suck the water from say the ocean up. However, if it got out of hand you would need a valve to slowly close the valve. In this way you could move water from the oceans (which is where octopi live) up into space and inside of a space ship like a mother ship which would be something like a giant Aquarium that you see places like Monterey Bay Aquarium and other places in the U.S. and around the world. So, you would get your sea water into space in an unorthodox way but it would be possible. Once you have your mother ship with a displacement of something like the Queen Mary you could go anywhere as long as you had your ATV type "Land Suits" available to go down to the earth in. You likely would take a minimum of water places (unless there were oceans on that planet to replenish the water. Then you would drain most of the water out except the minimum needed for life support of the Giant Northern Pacific Octopus flying the ATV Spacecraft back up to the mother ship.