A blow out tide is a low tide combined with wind blowing offshore to make an unusually low tide. These can drain harbors and estuaries, and be a problem for boaters. It can be pretty cool to see what been hiding under the water, tho.
It is often written as 'blowout tide' or 'blow-out tide'. Although the term is used by in National Weather Service warnings, it doesn't seem to appear in any dictionary, and I haven't been able to find what form might be the 'correct' one.