The marblemelon thing was more just comparing it to other farming plants, particularly the other indoor plants like potatoes and lantern fruit, which you get at the same time. The only real argument is for lantern fruit, if you have a little low-power outpost somewhere with a bioreactor, since you can just plant one tree in a pot and have 20+ fruits for some basic power. Melons still give about 50% more power per tile occupied in the reactor, but you have to replant them and such.
And nah, leeches were ruled out. There's just this weird, intermittent power-draining bug for the Cyclops now. Not sure what's causing it, but multiple people have noticed an inexplicable power drain on the sub. Only real way to secure yourself against it is the same as for leeches; remove all the cells from the engine before exiting the sub.
Not sure how people are completing the game without being infected, as you start the game with an infection level.
Actually, you don't! It seems the Kharaa bacterium is actually some sort of quantum disease, as its status is altered by observing it.
Basically, if you
don't scan yourself upon first building the scanner, you won't activate the first stage of the infection. If you don't listen to the Sunbeam's broadcast (and thus start the timer), you won't activate the second stage. If you don't push the button at the QEP, you won't activate the next stage (when warpers start attacking you) of the infection.
The only stage you can't really avoid when completing the game is
Stage 4, upon visiting the Lost River research facility
but it's entirely possible to stockpile almost everything you need to get yourself cured within a very short time of activating that stage of the infection, thus minimizing your time spent dealing with warper aggression.