The הגדה says:
במצרים לקו חמשים מכות, ועל הים לקו חמשים ומאתים מכות
In Egypt, they were struck with 50 plagues, and by the Sea [of Reeds], they were struck with 250 plagues
What were these 300 plagues?
Alongside Alex's answer, I would posit that they never had in mind specific identified plagues, and might even look at you strangely for trying to identify them. it strikes me more along the lines of 'kol ha-marbeh lesaper bitzias mitzrayim, harei zeh meshubach'; as well as what immediately follows these deductions in the haggadah, kamma maalos tovos laMakom aleinu. in other words, creative derashot + math to stress how much Hashem did for us. trying to identify each one would be missing the forest for the trees. Certainly so if Rabbi Yossi, Rabbi Eliezer, and Rabbi Akiva intended no specific trees.
R' Moshe Cordovero (Ramak), in his commentary Tefillah Lemoshe on the siddur, lists the following:
For each of the Ten Plagues - the respective manifestations of "af," "evrah," "zaam," "tzarah," and "mishlachas mal'achei ra'im" that were evident in each one - totaling fifty (as per R' Akiva).
The fifty plagues at the Sea. For some of them he lists sub-aspects too, but he says that since we don't have a detailed description of all of the events there, he's unable to provide the complete enumeration.
The Lubavitcher Rebbe brings down in his Haggadah, quoting the Rambam's Pirush Hamishnayos Avot Chapter 5 Mishnah 4, that while tradition tells us that there were many more plagues by the sea than in Egypt, there were only 10 types of plagues. They were all the same types of plagues which happened in Egypt, and at the Sea they split into numerous parts (how many depends on who you ask). The Rambam tells us that this is hinted to us from the verse in Shmuel 1 (Chapter 4, Verse 8):
"Woe is unto us! Who will save us from the hand of this mighty God? This is the God who smote the Egyptians with every sort of plague in the wilderness."
The Rambam says that "every sort of plague in the wilderness" refers to the plagues that happened in the desert by the Red Sea (since it is self-understood that Egypt was not a wilderness).
The Rebbe points out that this way the Haggadah doesn't contradict what it says in Avos deR' Nosson (Chapter 33), and in (at least) one version of the Mishnayos in Avot Chapter 5, that there were 10 plagues in Egypt, "and 10 plagues visited on the Egyptians at the sea" (The version of the mishnah found here, for example, does not have this line).