The swimming part...

[Selkie]

Agree. I had one session w/a coach who videotaped me. Pointed out some flaws in my technique of which I was completely unaware, and taught me how to correct them. Even for someone who swims purely for the love of it and is not a competitor/triathlete, the session was well worth the $$$$.

[JennK13]

Depending on what you're looking for, it isn't all that expensive. I paid $114 for a 3pack of training sessions at my local rec center. I paired with the tri coach, and the help I got with swimming has made a huge difference. I wanted one-on-one coaching for that, but they also offer swimming lessons pretty inexpensively, and also single training sessions for $40. I would have been fine with one lessons for swimming and using the skills trainings she gave me; I'm using the other sessions for other things.
I also took an open water swim clinic that was very helpful. The teacher gave a lot of useful information on swimming in open water and skill tips.
I don't think you need a coach, but a lesson or two would be of great value.

[GLC1968]

I swam on a team as a kid, so when I signed up for my first tri last spring, I figured it wouldn't take long to get it back. I was wrong. As a kid, I only did sprint swims and I was never any good at distance. As an adult, that was magnified because I was also out of shape (cardiovascularly) when I started. Plus, I'm competitive, so just surviving the swim didn't appeal to me...I wanted to do reasonably well.

I tried taking lessons at the pool where I swam. I signed up for adult lessons and I got a lot of 'you look great' and 'I don't see what else I can help you with, just practice more'. She would give me drills (good ones, actually) but when I asked what the drill was supposed to teach me or what it was for...she didn't have any answers (or, she made it up!). It was a total waste of time and money. I gave it 8 sessons (4 weeks) before I quit.

Then I signed up for a 10 week (1 session a week) clinic taught at the pool where my first tri was being held. It was a long commute on a weeknight for me, but it was 100% worth it. This guy knew his stuff and after 10 weeks, I was swiming my 500y tri distance. I set a personal record during the race, too (even though I did have to breast stroke some). Since then, I've only gotten faster with more and more practice (and races). I've not had any more lessons since the clinic, but I just swam that same 500y distance last week at the exact same pace as my first race except that this time, I was swimming it easy as a warm-up.

So yes, lessons can make a HUGE difference in your speed, your form and most importantly, your enjoyment of the sport. BUT, make sure you get lessons from someone who can help you. If the HS swim team member teaching at your pool can't help, get your money back and find someone else.