Any tips for feeling more comfortable while swimming in open water?
The best way is to swim in open water with a buddy as much as possible. But a lot of us don't have easy lake access. If a pool is your only option, get comfortable with bilateral breathing so that you can breath on either side depending on the waves & buoy placement. Doing laps with your eyes closed & then "peaking occasionally can simulate open water swim siting, too.
Not to be trite, but the most simple answer is "practice, practice, practice."
On a more practical level, most difficulty with open water swimming for beginning swimmers has nothing to do with swimming ability but rather the fear of the unknown. Conscious or unconscious, our minds wonder what is beyond the darkness.
To overcome this, come up with mental tools that will move you beyond the mental barrier. One way is to rationalize, "I have swam this distance in a pool no problem, I can do this in any water," or "my wetsuit is so buoyant, the bottom is irrelevant; I will float no matter what."
Another method is disassociation; remove yourself mentally from the environment. Imagine yourself somewhere safe or comfortable far away from the lake, ocean or river you are in.
Finally, another method is to get deeply involved in the swim. Focus on your stroke, your breathing, a spot on shore to show your progress; anything to get your mind focused on your swimming and away from the unknown.
Utilize one or more of these mental techniques whenever you are in open water. Hopefully, if you can, spend some time before your race in open water working on these techniques. If you need to, stay in shallow waters at first during your practice until you feel comfortable enough to venture into deeper areas.
Also remember, even if you experience a "panic attack" during a race, it is definitely OK to breaststroke or backstroke for some (or all) of the swim. You may find that having your face out of the water will make you feel safer.
As with all things, your comfort in open water will come with time. More than anything else, have patience. You will soon feel more and more relaxed in the water.
All the best!