Would you travel Croatia weather wise starting mid April or mid May?
If you want to swim I would go late May. My husband went at that time this year and went swimming almost everyday. He was up north, so down south would be warmer.
Ditto, the sea is slow to warm and only becomes tolerable to me in late May.
Otherwise, I’d still go with May because I detest cool weather.
Weatherspark says it's warmer and a lot dryer in mid-May.