Take a line and tie it to your main or jib halyard, and hoist it to the top.
Hold the bottom end of the line at the water line.
Mark it, lower the line, and measure the length. Since it's the hypotenuse of the triangle it will be a little longer than the air draft.
Add on any extra height for antennas or windex or anything else sticking up past the top of the mast.
.... assuming you can't find the answer anywhere else.
Be aware that when going under a bridge, waves can toss you up and down a few feet, and I've hit my antenna even with several feet of extra clearance.