Machshirin, Chapter 5, Mishnah 7. Our chapter revolves around a single question: when does water that has come onto an object count as having come there with the owner's approval, "bechi yutan," so that it can serve as hechsher lekabel tumah? Water that a person is pleased to have makes food susceptible to tumah; water that arrives while his mind is on something else entirely does not. This Mishnah lines up a series of cases, first several in which the water is unwanted, and then several in which the water is precisely what the person is after.
Water on a boat, its ekel, and its oars
"Hamayim ha'olim basefinah uva'ekel uvamishotos, einan bechi yutan." A man is crossing a river in his boat. Water splashes into the boat itself, or collects at the ekel, or wets the oars (mishotos). None of this water is bechi yutan. Regarding the ekel there are two explanations: the Rambam understands it as the anchor, while the Rav explains it as the spot at the bottom of the boat where water that seeps in through the cracks in the planking gathers and is soaked up. Either way, the reasoning is the same. A person has no interest whatsoever in getting his boat, his anchor, or his oars wet. He has no use for this water at all. In truth he would rather not have to cross this river in the first place, and he is doing so only because there is no other way to get where he needs to go. Water that arrives under such circumstances is not bechi yutan and cannot serve as hechsher lekabel tumah.
Traps and nets, and what happens when he shakes them
"Bametzodos uvireshasos uvamichmoros, einan bechi yutan." The same applies to the water on his traps and his fishing nets, the equipment a fisherman sets out in the water. He has no desire for these implements to be wet; his whole concern is catching fish. The water clinging to them, therefore, is not bechi yutan.
"Veni'er, bechi yutan." But if he shook the trap or the net out in order to get the water off it, the ruling changes. As we have seen in a number of cases in this chapter, once a person takes the trouble to shake off the water he has shown that the water matters to him; he has given it significance. That act makes the water bechi yutan, and it can now serve as hechsher lekabel tumah.
Cases where the water is exactly what he wants
"Hamolich es hasefinah leyam hagadol letzarfah." A man takes his boat out into the great sea (the Mediterranean, though the same would apply to any ocean) letzarfah, in order to strengthen it. The Rav offers two ways to understand this. It may mean that the boat's timbers had dried out and separated, so he brings it into a deep body of water where it will be submerged, the water will soak into the wood, and the planks will swell back into place. Or it may mean that he wants to test the vessel: to see whether it is seaworthy and will stay afloat in open water, so he takes it out to the sea to find out.
"Hamotzi masmer bageshem letzarfo." Here a person sets a nail outside while rain is falling, and his purpose is letzarfo, to temper it. A nail that has been left in the fire until it glows red is hardened when it is removed and immediately met with water; that hardening is precisely the result he is seeking.
"Hamaniach es ha'ud bageshamim lechaboso." Here a person leaves a burning brand outdoors while rain is coming down, his intent being lechaboso, that the rain should put it out. The ud may be a stick that is still alight, or, as the Rav explains, the iron poker used to stir a fire, which he places in water so that its heat will subside.
In all of these final cases the person is genuinely pleased with the water: he wants his boat submerged or tested, he wants his red hot nail hardened, he wants his brand or poker quenched. Because the water is precisely what he was looking for, it is bechi yutan, and it is fit to serve as hechsher lekabel tumah.