Swimming…

It may crash later (exersize). Did you disconnect your pump from your body before swimming?

I do. I then check my bgl and "top up" every 2hrs or so, then I can see what my bgl is doing.

It's not too drastic even tho you feel crap currently

When I train hard I raise to 21 !! An hour after that I drop to 4.5. No bad feelings.....

Swimming is same as HIIT if done intensively. That's a 'good' BG spike as I was explained by my endo. Your body definitely gets more benefits than eventual damage. 😉 In many cases it drops back down. And if not, just a small correction is needed.

The adhesion of the sensor if you decide to start swimming regularly - that might be a challenge

Random question... was it a heated pool?

Swimming drops me to 70 and keeps me around there for over a day even with minimal insulin use.