The only thing I had for myself has gone
I know I should feel good about my wife showing an interest in swimming in the sea. It's nice. It's something we can do together. And a marriage is always helped by common interests and wanting the same things and having similar aspirations, which we do in general.
Specifically on the sea swimming, a bit of goal congruence should be a good thing. For example, it is in my interests that when thinking of going on holidays, my wife is invested in wanting to go somewhere near a swimmable sea. And sometimes, in summer, it's nice to have the whole family at the seaside.
So like an idiot I encouraged her. My wife would be a summertime bather in general. But I decided to convince her that if you could do it in summer you could do it in winter as well. The temperature difference isn't huge sometimes. So, lo and behold, in she got, and she took it in her stride. She took my advice of just walking in, no drama, and telling yourself it's all in your head.