I have experienced two types of swim starts.
The first was where the fastest female went first and then the next slowest ten seconds after that and so on until all women were in the water, then the slowest man until all men were in. We swam 3 lengths in one lane, then went under the ropes and did three lengths in that lane and so on until we had 'crossed' the pool to get out. This way was a little scary as if you were a slow female swimmer, you tended tog et swum over by the slow men.
I did another one last weekend where there were four of us per lane (ie 24 people in the pool at one time). We were set off ten seconds apart. The next wave did not start until we were all out of the water ie the pool was empty. This appraoch worked really well as there was little chance of being swum over.
Which Tri are you doing? You could either post here or approach the race organisers to find out which way the handle the swim