In general, the Halacha permits traveling by boat on Shabbat, provided that one boarded the specified time before Shabbat. However, the Shulhan Aruch (248:2), based on a responsa of the Rambam, adds a caveat to this practice. He rules, that it is only permitted to travel over water on Shabbat, if it is not known for certain that the depth of the water is less than 10 tefachim (handbreadths). If it is known that the water is so shallow, travelling presents a problem of Tehumin (Leaving the boundaries of Shabbat.)

Accordingly, even if one is in doubt as to the depth of the water, it is permitted to travel. Since the prohibition of Tehumin is M’drabanan (Rabbinic Law), the principle of Safek D’raban L’Kula, (“A doubt in a rabbinic matter is resolved leniently”) is applied.

Rabbi Akiva Eger (1761-1837) extrapolates a fundamental Halachic principle from this precedent. He points out that even travel above 10 tefachim is subject to question. The Talmud already defined such a case as a Halachic doubt. Therefore, one could have argued that travel over water whose depth is unknown, should be prohibited because of a “Safek Sefeka” (Double Doubt) of stringency: If the water is less than 10 Tefahim, it is certainly prohibited; if the water is deeper than 10 Tefahim, it still presents a problem, because of the side that is stringent even in such a case.

From the fact that the Shulhan Aruch did not take this approach and maintained a lenient position, it can be concluded that the aforementioned argument is faulty. Rabbi Akiva Eger, based on the Shaar HaMelech, thereby deduces a general principle that in a doubt with regard to Rabbinic law, even a double doubt to be stringent is resolved leniently.

This principle is adopted by Hacham Ovadia in Yabia Omer, as well as in Hazon Ovadia (Hilchot Shabbat, Vol. 1). He defends this leniency as a general rule in all areas of Halacha, against those opinions that view it as a special ruling in the case of Tehumin.

SUMMARY
A Halachic doubt in a Rabbinic Law is resolved leniently, even if there are two doubts towards stringency.