Too cool for swimming in the ocean at Santorini & Rhodes in April ?
 

My kids are water nuts and wanted to know if it would be warm enough to surf in April, I can't think of a better place to find out....over to you !

Yes, it is too cold in April and May too for swimming but you could serf in the Arctic in April if you are properly dressed.

Swimming in Crete is possible in May.

Several years ago we did an Agean cruise during the summer--I think July. We got in the water at Mykonos and I thought it was cold. However, I am from Texas and accustomed to hot weather and warm water. This is a beautiful area and kids are usually not as particular about water temperature as grandmothers. Have a great trip.

April will probably not be warm enough but kids are more open to cold water. The earliest I have gone in was the last week of April because it was hot and the water cools you off. I have gone in lots of times in May but then I am from Canada.

Aside from the question of temperature, which previous posters have covered, your kids should be aware that the Greek islands are not in an ocean, but in the Mediterranean Sea, which doesn't have any tides and so doesn't normally have the sort of conditions they may be accustomed to surfing in. They may be a bit disappointed if they are expecting Atlantic-style rollers.

Swimming is eminently doable in late May in the Cyclades, but as WhoAmI says, there is no surf in the Aegean.