When we exercise, our bodies do two things affecting our diabetes:
- our livers release glucose to give us extra energy.
- we become more efficient at using insulin so need less
It is the balance between these two reactions which determines whether our BG goes up or down.
There are some typical rules of thumb but they don't always happen.
For example, longer exercise usually results in a lower BG. Short stop/start exercises usually result in higher BG.
However, as I am writing this, I wonder if you know that the exercise is causing the rise.
Have you tried taking your fasting reading before getting in the pool out of interest?
The rise may be related to dawn phenomenon (liver dump to give you energy to start your day).
It could be useful just to discount this as the reason.