Relax. I can't stress enough how important it is to relax in the water especially during an open water swim. With all of the chaos going around you, I know it can be difficult. But this is where the phrase mind over matter comes into place. If you are not relaxed your breathing will become faster and you could start to panic. Close your eyes and clear your mind.
You are allowed to hold onto a boat. Yes it's true! Most races have spotters in the water on kayaks or watercraft. You are allowed to stop and hold onto the boat. But the key is you must stop and the boat has to be be stopped as well. You cannot let them pull you along.
Start in the back or off to the side. When that gun goes off the swim start can be really overwhelming. Arms are flying people are swimming on top of people and it can be scary! To avoid some of this swim chaos I would suggest starting in the back of the swim line-up or off to the side. It is usually less crowded in these areas and you shouldn't find the fastest swimmers here. Also one thing to keep in mind is that the buoys can get crowded. So if you have an out and back swim usually on the inside of those buoys you'll begin to experience the "washing-machine" experience once again. My advice is to just take a deep breath and try to pull to the outside if you can.
Test your gear. If you are going to wear a wetsuit make sure you practice this ahead of time. Swimming with a wetsuit is very different and you want to be comfortable. Make sure your goggles fit correctly and don't leak. Also one thing to keep in mind if what type of lens you want for your goggles. If you are doing an open water swim that heads directly into the sun, then you want some sort of tint to your lens.
Have you ever done an open water swim? What basic tips would you suggest for a first-time triathlete?