Yeah! I know swimming "twice" the distance sounds long, but that extra security will make the 250m seem downright doable in the conditions of the race.

And, if you continue in triathlon, a lot of the sprint distance races have .5 mile swims, which are ~800 meters, so you could continue to build to that if you decided to do another race after your first.

You might also want to shorten your last Thursday swim before your race to something like 200-300m, to have kind of a mini "taper" - to go easy on your body in the few days before the race. You can plan this into your cycling and running, too.