Worst injury during swim?

The wetsuit will definitely help your buoyancy and sleeveless makes it easier to move your arms.  If you're fast now, you'll probably be faster in the wetsuit.  Practice swimming in it a few times before your race so you can get used to it without the race scrum.  Also, practice your transition from the swim to the bike so you can figure out how best to get out of the darned thing in under 10 minutes.  As for whether or not to wear it at the lake, consider the water temperature.  If it's a USAT-sanctioned race, it's legal to wear it if the water temperature is below 78 degrees.  If you can tolerate cooler/cold water, you could do without it.  Your best bet is to give it a try and make up your mind based on experience.

You may be in the wrong sport if you want to "mix it up".  The objective is to achieve the fastest time to the finish, and the best way to achieve that is generally to not waste precious seconds sparring with other swimmers.  Also, assuming you're in the U.S., and depending on how aggressive you get, it could be a violation of one or more of the following rules:

3.4 Race Conduct

b. Unsportsmanlike Conduct. Participants shall refrain from unsportsmanlike conduct, including the improper use of language or conduct directed toward an official. Violations of this Section shall result in disqualification or a variable time penalty depending on the severity of the violation as determined in the sole discretion of the Head Referee.

c. Obstruction. Participants shall refrain from intentionally or accidentally blocking, charging, obstructing, or interfering with the forward progress of another participant. Any violation of this Section shall result in a variable time penalty.

g. Unfair Advantage. No participant shall use his body, head, arms, or legs to gain an unfair advantage, or to push, pull, hold, strike, or force through one or more participants. Any violation of this Section shall result in a variable time penalty.

l. Endangerment. No participant shall commit any dangerous act which could cause injury to any participant. Any violation of this Section shall result in disqualification.

ETA:  To answer your question directly - I've been in races where people have died from injuries they've sustained from accidents during the swim and bike.

Contact can be expected. Punched and kicked should not. That does not make you feel like a triathlete (well, not for me at least) I would suggest if it is your first race with no OWS experience that you do not "mix it up with the pack" and ruin someone elses swim. Go and have fun and race hard, but race smart. Triathlon is not supposed to be a contact sport.

Curious as to the intentions of this thread, asking for worst horror stories. 99% of the time, contact on the swim is unintentional and harmless. Expect to get bumped or even groped by a passerby, but by in large it's just swimming. People will be faster than you, people will be slower than you. Be nice and race your race.

The AC race you are referring to I am guessing is Tri AC.  It's a time trial start and only 5 go off at a time.  It cuts down on the chaos.

It's more of a lateral motion, like when someone realizes that they've gone off course and then start swimming at a diagonal, not realizing that they are now going way off course in the other direction, and you're stuck in the middle. It's happened to me in many many races (the one getting swam over, not the one pinballing all over the place.) It also happens a lot at the buoy turns when everyone gets gridlocked

The swim-over happens unintentionally, but surprisingly isn't rare, unfortunately.

Most often, it's due to different waves starting at different times. Even if you're not a super-speedster, it's easy to be busily trying to draft some fast feet ahead of you, then -whammo! you run into the last swimmer from two waves ahead, whos' going literally 1+min/100m slower than you.

I used to also never believe you could unintentionally swim over people like this but having accidentally done this a few times as in the past few years, I'll say it can be hard to avoid. In all cases where I've accidentally done it, there were bodies boxing me in on all sides, everyone was going at the same speed, and then we ran into the next wave. Next thing I know, I'm on top of someone, and can't even stop to say SORRY! since everyone else around me is so scrunched up. Even in retrospect there was really now way I could ever see that slower swimmer ahead while drafting and keeping up with the scrum of folks in my AG.

I'll also add that if you the hand of a FOP swimmer lands on your back, it'll likely feel like a punch and/or a hard grab, as these folks pull pretty hard. It's not intentional, but I've seen on this forums and elsewhere, some 2:00+/100yd pool swimmers who misinterpret this as intentional, probably because they can't imagine their pull being this forceful. 

(I've also been swum over a few times in my early years, so I know how it is to be on the receiving end, including one memorable experience where literally like 5-6 speedy women in a row from 2 waves back who were tightly drafting in a lane, plowed right over me by accident, and yes, it felt like each and every one of them was trying to kill me, when they obviously weren't!

This.

I used to play Div. I NCAA water polo, so I can tell you about blood and broken bones (including my own) in the water.

In tris, not so much.   Wanting to get beaten up implies wanting to beat up.  See bold above.

It's not usually someone way faster than you.  It's the who get on your feet, then grab an ankle and pull.

I strongly disagree with this. Again, you might feel it's someone intentionally grabbing your ankle, but there is no reason whatsoever anyone has any motivation to do this, even if they're trying to go around (or even over) you. For anyone trying to go faster, it would be a complete waste of time and energy to grab an ankle.

The grab comes from the palm down position of a hard pull, like a velcro hook. It's not that they're trying to grab you at all.

My biggest fear is breast strokers - as long as everyone is trying to swim properly then the contact isn't a big deal.  But those breaststroking feet just come out of the gloom and threaten your fingers.

My wife saw a guy walk back out of the lake at IM Austria five minutes after the start cannon having been hit by a breastroker.

It's not illegal and I know a lot of weak and unconfident swimmers need to use the stroke but for me it's my biggest fear in open water.  Especially in wave starts where you can catch up the slower swimmers in the wave ahead. 

I am usually the wave in front of the ex-collegiate swimmers (those speedy male 25-29 fishies) and many times someone in that wave will come cruising up behind me and their reach is SO LONG and aggressive that at the time it certainly felt like something very intentional (and actually funny for some reason). But as I watch those guys swim more and more I realize it's just the way they reach and pull. So I agree that while they might not be trying to grab, it can very much feel that way in the water.

I've made a lot of swim progress of years, so I hear exactly what you're saying.

When I was a BBOPer and got grabbed/swum over, even though I was warned about it, I couldnt help but thinking WTFFFF??!?!? In retrospect, it was because I had no idea how forceful and strong these stronger swimmers were - I literally couldn't even fathom it, as that would have been greater than my max effort for a single pull, so it seemed so intentional. 

I'm significantly better now, and it's amazing how much more forceful and faster my catch is, and I'm not even podium-fast, so those guys are much more powerful. I would hate to have one of those guys landing a stroke on my back - yeouch!