The Mishna in the last chapter of Masechet Berachot teaches that upon seeing rivers, one recites the Beracha, "Oseh Ma'aseh Bereshit"(Maker of Creation). Rabbi Yehuda says that upon seeing the "Yam HaGadol" (the great sea), one recites, "She'asah Et HaYam HaGadol" (Who Made the Great Sea).
The question is what the "Yam HaGadol" refers to. Maran, in 228:1, based on Rabbenu Yonah, states that it refers to the waters that flow between Israel and Egypt, i.e. the Mediterranean Sea. This is how the Mishna Berura understands Maran, as well. The commentaries explain that while the Mediterranean may not be the largest body of water, but because it's connected to Eres Yisrael, it becomes "Great" by association.
Accordingly, one who has not seen the Mediterranean within thirty days should recite the Beracha with Hashem's name. However, the Bet Yosef cites the Teshubot HaRosh who understands that "Yam HaGadol" refers to the "Oceanus," meaning the Atlantic Ocean.
Thus, there is a Machloket whether this Beracha applies to the Mediterranean. Because of this Safek (uncertainty), Hacham David rules that when seeing the Mediterranean, one should incorporate both Berachot by saying, "Baruch Atah Hashem Elohenu Melech HaOlam SheAsah Et HaYam HaGadol-Oseh Ma'aseh Bereshit." Even if the Mediterranean is not the Yam HaGadol, it is no worse than seeing other rivers, whose Beracha is "Oseh Ma'aseh Bereshit." If it is Yam HaGadol, the correct Beracha was said. If it's not the Yam HaGadol, then the correction was made "Toch K'deh Dibur"-immediately. This circumvents the problem of Safek Berachot.
According to this, if one sees the Atlantic Ocean and the Mediterranean Sea simultaneously, e.g. from Gibraltar, there is no Safek, as one of them is certainly the Yam HaGadol. In that case, the Beracha of "SheAsah Et HaYam HaGadol could be made with Hashem's name.
Those who live on the East Coast of the United States who see the Atlantic Ocean, even when driving, within a thirty day period, do not recite a Beracha.
SUMMARY
One who sees the Mediterranean Sea for the first time in thirty days, recites the Beracha: "SheAsah Et HaYam HaGadol-Oseh Ma'aseh Bereshit."
