If you can understand somebody's speech, you can understand a language you do not know
This can be shown through similarly worded features like the Monk's Tongue of Sun and Moon and the Shepherd Druid's Speech of the Woods:
[...] Moreover, any creature that can understand a language can understand what you say. [...]
[...] In addition, beasts can understand your speech, and you gain the ability to decipher their noises and motions. [...]
These are clearly intended for you to be understood even if you do not share a language. Oceanic Soul uses similar (if not identical) wording, so I would conclude similarly that you can be understood even without sharing a language.
I believe this also makes sense given the natural meaning of the phrase. If I could suddenly understand Japanese I would actually know the meaning of the words I hear, not just where to put spaces or some other vague thing about the language.
Features with language-based restrictions state those restrictions
For example, the Great Old One Warlock gets the Awakened Mind feature:
[...] You don't need to share a language with the creature for it to understand your telepathic utterances, but the creature must be able understand at least one language. [...]
The tongues spell states:
[...] Moreover, when the target speaks, any creature that knows at least one language and can hear the target understands what it says. [...]
The Ghostwise Halfling's Silent Speech feature states:
[...] The creature understands you only if the two of you share a language. [...]
From these, we could conclude that Oceanic Soul thus does not have language restrictions when it states:
[...] any creature that is also fully submerged can understand your speech, and you can understand theirs. [...]
Therefore, there is no need for a shared language. We can say, however, that the creature must be able to speak in some way in order for their speech to be understood in the first place (an exact definition of what counts as "speech" can only be given by your GM). If the creature were required to speak in a language you actually know, the feature would state as much.
Some features (as far as I can find they are all telepathy features) include extra details. Quoting the Awakened Mind feature again:
[...] You don't need to share a language with the creature for it to understand your telepathic utterances [...]
This is explicitly called out as being the case and it is not explicitly called out in the Oceanic Soul feature. One could argue the exact opposite of what I did above and that because Oceanic Soul does not include such a detail, it does require that you share a language, but I would simply chalk this up to telepathy being an especially unusual method of communication.
