In lakes rivers and the sea the water moves around at an alarming rate.
In rivers, yes you get currents, but except in proper good moving water, they rarely make you go sideways relative to the river direction.
In the sea, you sometimes do, although I have a feeling triathlon events are typically timed to be at the turning of the tide (and avoid places with rip currents etc.), so very limited currents.
In lakes though? Currents? I’ve swum in a lot of biggish and small lakes (big ones in the Lake District, in the Alps, smaller local lakes), and I’ve never experienced a current that was strong enough to make even a slight difference to the way I was heading. I know there are convection currents etc. in lakes, but they are typically very slow, and make no difference to swimming. Obviously in lakes with rivers coming in and out, there are currents where the rivers enter / exit, but other than that, I’ve never experienced lake currents.
Wind now, that is a different matter, swimming in a strong side wind is a pain, although if the wind is enough to knock you off course, it’d be obvious and you’d know about it before you got in the water.