I’m not sure what your first sentence means sorry, but I’m guessing you want to swim in the sea but are a little scared? My sister goes wild swimming everywhere she can, rivers, quarries, sea, in all seasons. I actually think she is addicted to it.
I, only once, swam in a river with her, it was terrifying but exhilarating, going against the current and seeing huge fish swimming underneath us. We were just in cozies, and I got very cold when I had been out a while.
I would say go for it, you are in a group, make sure you have lots of warm clothes when you get out, and enjoy it. The health benefits are amazing, it can even stave off dementia, by resetting the brain somehow as your body temperature lowers. (I don’t know this for a fact, but I read it on some medical website).
Good luck, go for it!