Oyogimasu = I swim, I will swim.
To say “I am swimming” (at this current point in time), you need to use the continuative form, which is the te-form of the verb plus imasu (or iru in plain form). Not entirely sure if you’ve learnt the te-form yet, but for oyogimasu it’s oyoide.
Oyoideimasu = I am swimming
“Tsukimasu” doesn’t mean “forget” – you sure you’ve got the right verb there?
Interestingly enough, your second question is also covered by the same grammar point. For instantaneous verbs (i.e. verbs that happen once and leave a changed state behind, like getting married, or forgetting), the te-imasu form means “is in the resultant state”
Kekkon shimashita = I was married (i.e. the ceremony was last week or whatever, but whether I’m still married is unclear)
Kekkon shiteimasu = I am married (i.e. the ceremony was last week or whatever, and I’m still married)
Kekkon shimasu = I will be married