Duo should be a bit more precisely in translating. Wir schwammen im Meer = We swam in the sea. / We swam in the ocean = Wir schwammen im Ozean. das Meer and der Ozean, yes Duo, there is water in there, got that, but however, these are two DIFFERENT concepts, aren't they ??!
No, "das Meer" can mean either "the ocean" or "the sea". It is interesting that that in German that the names of some oceans will use "Meer" and others use "Ozean" and the names of some seas use "Meer" and others use "See". http://dictionary.reverso.net/english-german/ocean http://dictionary.reverso.net/english-german/sea
The preterit for 1st person singular is „ich schwamm“. In perfect tense it's „ich bin geschwommen“. You can take a look at all inflections here: http://en.wiktionary.org/wiki/schwimmen#Conjugation
But this sentence requires the 1st person plural pronoun wir/we.
As for the preposition:
im is the short form of in dem, used for the dative case. in with dative case has the meaning of already being within something and not leaving it.
ins is the short form of in das, used for accusative case. in with the accusative case has the meaning of not being in something at the start but moving into it. „Wir schwammen ins Meer.“ translates to „We swam into the ocean.“
Yes, im is short for in dem, which is used for masculine and neuter nouns. But the noun displayed to me in German is Meer, not See.
das Meer (neuter) translates to sea in English
der See (masculine) translates to lake.
There is also a feminine die See, which can indeed translate to sea or ocean. But apart from some expressions in seaman's language (e.g. die raue See – rough waters), I would consider the feminine variant to be outdated. If you encounter a See in the wild, it's usually the masculine one.
There is no feminine or masculine Meer in German, as you wrote “die Meer” and “der Mer”, which I consider to be a typo.
"die See" = "the sea"
"der See" = "the lake"
"das Meer" = either "the sea" or "the ocean"
"der Ozean" = "the ocean"
Scroll up for the dictionary links which shows names of seas and oceans which would have to be memorized to know which word is used with each.
In English, when you are referring to all the salt water that covers the earth, you could say either "the sea" or "the ocean", but when you are naming specific areas of that, one or the other is used in each particular name of each area. We say "the Mediterranean Sea", but we say the "Atlantic Ocean". Most seas are enclosed at least partially by land. Seas are often smaller than oceans. https://en.wikipedia.org/wiki/Ocean https://de.wikipedia.org/wiki/Ozean https://en.wikipedia.org/wiki/Sea