You drop the ता for the present continuous tense.
मैं तैरता हूँ - 'I swim' - indicating a habitual action
मैं तैर रहा हूँ - 'I am swimming' - indicating that the action is happening now
मैं तैरता रहा हूँ - 'I have been swimming' - indicating that the action started in the past and is continuing now
Note: The present perfect continuous (third sentence above) is not used much in Hindi and we usually use the present continuous (second sentence above) instead, if there is no scope for ambiguity.