板子一枚下は地獄
(Itago ichimai shita wa jigoku;
“One plank below is perdition”)

Definition:

This sailors’ saying expresses the fact that sailing is a dangerous profession, and warns them to be careful while out on the water. Only a thin bit of wood separates you from what is likely to be your death if that protection ever fails. By extension, it can metaphorically describe a situation where a single error can result in disaster.

Breakdown:

We begin with the compound noun 板子 (itago), a plank that is specifically part of the flooring of a wooden boat. This is followed and modified by number-counter 一枚 (ichimai), “one flat thing,” and then associated (despite the lack of particles) with the noun 下 (shita), “down,” “below.” All this is marked as the topic of discussion by the particle は (wa), and the comment on this topic is the pithy compound noun 地獄 (jigoku), “hell.”

Notes:

In contrast to the unitary, eternal hell of Christian theology, Buddhism conceives of multiple hells, each with its own degree and style of awfulness, which pay off bad karma before the soul moves on to a different plane of existence. My sources don’t explain why the sailor is necessarily bound for a bad plane in the next life; perhaps it’s simply because a saying about how a finger’s breadth of wood is all that separates one from a pleasant afterlife wouldn’t make a very good warning.

One thing to be careful of is that an 板子 does not seem to be part of a ship’s hull; as near as I can make out, it’s either the part you walk on, and/or a trap-door in that floor that can be opened to provide access to the bilge compartment, at least on larger ships. Thus, the “plank” in question is not necessarily the part of a boat that keeps the water away, but rather the part that supports the sailor directly.

Another thing to be careful of is that read as Itako, 板子 appears to be a family name, although hardly a common one.

Variant phrases tend to specify that what separates the sailor from “hell” is a thickness of just 三寸 (sanzun), about 9 cm or 3.6 inches – or just 一寸 (issun), which is obviously one-third of that. Not all versions mention the physical board itself, and some merely call it by the generic term 板 (ita).

Example sentence:

「ね、一緒にあれ見ない？『ベーリング海の一攫千金』っていうアメリカの番組」　「嫌だヤダ！私、かなりの海洋恐怖症だよ。板子一枚下は地獄って考えるだけでお腹が痛くてしょうがない。絶対見たくない！」

(“Ne, issho ni are minai? ‘Beeringu-kai no ikkaku sen kin’ tte iu Amerika no bangumi.” “Iya da yada! Watashi, kanari no kaiyou kyoufu shou da yo. Itago ichimai shita wa jigoku tte kangaeru dake de onaka ga itakute shou ga nai. Zettai mitakunai!”)

[“Hey, you want to watch that show together? The American one, Deadliest Catch.” “No way, no! I’ve got a really bad phobia about the sea. It makes my stomach hurt so bad I can’t stand it, just thinking about how there’s just a plank between you and a watery grave. I definitely don’t want to watch it!”]