Taken from german.about.com:
Helping Verbs In English, the present perfect is always formed with the helping verb "have," but in German some verbs require "to be" (sein) instead. There is a rule for this condition (see below), but it is best to simply memorize the few verbs that usually use sein as a helping verb. (Most are intransitive verbs of motion.) These verbs include: bleiben (stay), fahren (drive, travel), fallen (fall), gehen (go), kommen (come), laufen (run), reisen (travel), sein (be), steigen (climb), sterben (die), wachsen (grow), werden (become). Example: "Er ist schnell gelaufen." = "He ran fast."
I've checked with a large number of my friends (most with post university level study) and all agree that, "You have SWAM in the sea" is just as correct as, "You have swum in the sea". Obviously not correct if you check official conjugations, but in reality, it has become such a common error that it is for all intents and purposes a legitimate way to conjugate this sentence now.
There's no need for argument here. "It's I have swum" or simply "I swam." Duo shouldn't change anything because this is the section on perfect present. All anyone needs to say in the comments is "for the record, this verb is almost always used in the simple past tense." Honestly, I'm pretty sure every native speaker on here has heard "have swum" many times. If they didn't go to elementary school and have never opened a book, why would they be trying to learn a second language? However, it's entirely true that "swam" is much more common.
And no, don't say "I have swam." Rarely used conjugations might sound slightly odd but they still sound much better than incorrect ones.