[color:"pink"]>The water temperature on the lake where I sail is
>currently 58.64 degrees ten feet below the surface.

>Are these temperatures I should worry about if I'm
>sailing without a wet suit?[/color]

The quick answer, Yes

The long answer, It depends.

This is the topic of much discussion among open water paddlers. Much has been written about it due to the fact that extended emersion in cold water is the number one cause of death in for sea kayakers.

If I were sailing with crew on a protected bay with lots of other boat traffic, winds in the 5-7 knot range, air temps in the 70's to 80's with abundant sunshine, I might decide to forgo the wetsuit for the outing. Last Sunday, however, the air temps in Vermont peaked at 30 degrees, there wasn't a boat on the water (Lake Champlain), and I was sailing single handed. The water temp was in the low 50's. Winds were light and variable, yet I wore a full-length drysuit and knee-high neoprene boots.

There are no hard and fast cutoff points when it comes to dressing for the borderline conditions you describe. My advice, however, would be to dress for the water temperature (not the air). If in doubt, put on what you might consider appropriate for the day and go jump in. Swim around for a while. If you're not comfortable in the water, you'd better re-think your insulation. The other factor is the risk of extended emersion. The greater the risk, the better prepared you should be.

BTW, we're white now in Vermont and the boat was put away until Spring following Sunday's outing. For the next several months I will read with envy the posts of those who sail through winter.