Planning an open water swim - how to set up different waves

A friend is hosting a 1 mile ocean swim in the Atlantic Ocean next month with approximately 250 swimmers aged 9-90. Since the swim has grown over the years, having a mass start has proven unwieldy and he'd like to break up the swim into different waves. Does anyone have any ideas how to divide up swimmers into different waves? In this case, we don't know the speed of the majority of the swimmers so sending the fast swimmers off before the slow ones (especially due to the large number of beginners) would be difficult to do in advance. Each wave would be given different colored bathing caps and unique numbers (e.g. Wave 1 is 101-199, Wave 2 is 200-299 etc). I was going to propose having an under-18 wave and a 19-up wave, which would reduce the number of swimmers in each wave somewhat, but any other suggestions are welcome. Thanks in advance!