Your best bet is to look up last year's results for the race you have in mind.

Bear in mind that wearing a wetsuit, the race environment, and the ability to draft can all help your speed. On the other hand, rough water, choosing to swim a wider course, and the presence of other swimmers to contend with can slow you down. For these reasons, it can be hard to draw a direct comparison between swim times in the race versus in the pool.

If you are well-prepared and can comfortably swim the distance, you should be fine.