when the water temps hit 75
That (75F) would be considered unsafe scalding temps by some here in Ontario Canada. Summer in Ontario starts once there is only remnants of dirty black snow in roadside ditches. I hate it when my golf ball ends up in some snow and you lose it, it costs you a stroke. But to be serious for a bit I did not use my shovel this past winter, compared to the previous 2 winters of Artic temperatures and 30 foot high snow drifts along the Erie lakeshore we needed a break. We didn't even need to go to Cuba this year, we usually go twice every winter to break it up. It is not uncommon to golf before the end of March here in southern Ontario. We do consider summer starting April 1st and ending late November. I have to shake my head sometimes when I see Sandals, T shirts and shorts and there is still some snow in ditches and some ice still on the lake and I have a toque on.
I finally get to the query and as far as asking someone when it's warm enough to partake in water sports just do the plum test for yourself. Dip the plums into the body of water in question and if they turn plum purple you may want to wait a bit.
Depending on the time one is in water and individual metabolism some individuals can show signs of hypothermia in water temps below 70F. I have felt water temps just a few feet below the surface much colder than the reported surface temperatures many times when snorkeling. Current plays a large part. We have swam in 60F water, not for long though.