Many triathletes report that they don't get huge time gains for the first year or even 2 with TI, but that they come out of the swim in better shape than before, which allows them to get better times overall. That is because of a more efficient less energy-wasting stroke.
Eventually, you will get time gains on the swim portion as well, but don't be discouraged if you don't make that 25 minute goal. That is doable, but maybe not right away. 27 minutes would be respectable as well.
The swim is the shortest segment of tris so it isn't important to shave time off there, but it can launch you to perform better at the next 2 legs.