So I'm going to make a list of things I consider vital for your first swim. The different seasons obviously affect what you might want to wear in the sea/loch/lake that you are going to swim in. as does your own personal preference. Some folx are naturally cold beans and so therefor swim in wetsuits year round. I am a hot potato, and so am currently swimming (still mainly splashing around, although my range of motion is improving, I still don't want to push it too too far) in swim shorts. Whereas earlier on in the year (pre-surgery) I was swimming in a short wetsuit. So with that in mind, I would say to take a swimsuit of choice. I will also point out something about the neoprene on our friend the wetsuit - it's the same material a lot of binders are made from and so might help those of you who have chest related dysphoria.
Also take a warm drink if swimming over the winter. Some would say to bring a hot drink whatever the season/weather. It's to stop what is known as after drop (https://loneswimmer.com/2019/12/08/how-to-what-is-afterdrop/) but again, dear reader, I am a hot potato and so therefor did not need this. I also started back when the weather was a lot colder and I had been doing the Wim Hof technique in order to help me acclimatise (https://www.wimhofmethod.com/) and so I have not had the same need as some. I'm also rather overweight which means I'm warmer in the water than some.
I would invest in some sort of changing robe (if you have the money, sure go for a Dry Robe). Personally, I manage just fine with a microfibre generic robe. The reason they are good is largely because you have something that you can easily get changed under - there's an issue with public indecency otherwise! I don't mind naked bodies but others might take umbrage. That being said, I do secretly want to go skinny dipping at some point, but that's by the by. So yes, look in swim shops and so on for the many change robes that are available. This way all the dysphoria inducing bits are hidden, not just if you're trans, and you are less likely to flash people.
Lastly, one of my favourite things, especially for solo swimming, is a tow float with a dry bag. This is because you can pop your valuables in it as well as having something that makes you visible from the beach as well as giving you a floatation device should you come in to any difficulties. Or you can just hold on to it when you get tired. Either way, this one is one that I would say is a must buy for distance swimmers or people like me who just like to make sure their keys and phone are safe while they swim!
This is my friend Ellie. She will appear in many pics of me in the sea. This is largely because she's my main swim chum at the moment (I am moving away and so that will likely change) and is better at selfies than I am.