Mikvaot, Chapter Six, Mishnah Five
1) A chest or a box which is in the sea: one may not immerse in them unless they have a hole as large as the tube of a water-skin.
a) Rabbi Judah says: in the case of a large vessel [the hole should be] four handbreadths, and in a small one [the hole should be as large as] the greater part of it.
2) If there was a sack or a basket [in the sea], one may immerse in them as they are, since the water is mixed together.
a) If they are placed under a water-spout, they do not make the mikveh invalid.
b) And they may be immersed and brought up in the ordinary way.
Section one: The chest or box is floating on the water and water is entering through cracks. The question is whether the water in the box counts as if it was in the sea, in which case there is certainly sufficient water to immerse in it. The first opinion rules that a hole in the box the size of the tube of a water-skin is sufficient for the water in the box to count as mixed in with the waters of the sea.
Rabbi Judah rules more stringently. He says that if the box is large, meaning that the majority of the box is more than four handbreadths, then the hole must be at least four handbreadths for the water to be considered mingled. If the box is smaller than that, than the hole must be at least the greater part of the size of the box. Basically, the box must have a pretty big hole relative to its size.
Section two: The sack and the basket are not at all water-proof, therefore their rules are different from those governing the box or chest. If they are in the sea, they count as being part of the sea and they don’t need a hole in order to be able to immerse in them.
If they are placed under a water-spout they don’t cause the water to be considered “drawn water” as would vessels that are made to hold water.
Finally, since the water in them doesn’t count as “drawn water” one could immerse them in a mikveh containing exactly forty seahs of water and then pick them up again. The water that will gather in the basket and then fall back into the mikveh does not count as “drawn water” which would disqualify the mikveh.