Not necessary. The author’s theory allows for creatures that could cause environmental damage to get into the tanks. But the author believes that when those creatures are deposited in a different environment, they’d die before actually causing significant environmental damage.

Not necessary, because the author doesn’t suggest anything about the specific conditions under which tanks could be emptied and refilled in midocean. If this could occur under conditions other than calm air and flat seas, that doesn’t undermine the author’s reasoning.

Not necessary, because the author’s argument isn’t based on what has actually happened in the past. The author is theorizing about a potential solution based on where midocean creatures and coastal creatures can survive. This doesn’t require any assumption about what has rarely occurred or commonly occurred in the past.

Not necessary, because even if there are other times when the tanks are emptied and refilled, that doesn’t undermine the author’s reasoning. The author happens to be talking about a problem resulting from emptying/refilling the tanks when unloading/loading cargo, but if emptying/refilling occurs at other times, the author’s solution could still address any problems resulting from sea creatures getting into the tank during those other emptying/refilling occurrences.

Necessary, because if it weren’t true — if there are NO oceangoing ships whose stability could be adequately maintained while emptying/refilling in midocean — then the author’s solution is not a “viable” way of addressing the problem. If oceangoing ships could not maintain adequate stability by using the author’s proposed solution, then it’s not a realistic option.