Re: Do you go ou with a 30% chance of rain?
Great question. First, if thunder showers are predicted then I will only go out if they are predicted to be scattered and the type that are not associated with lightening and no sudden gusts especially when they say gusts are expected to be 40-60 mph range

.
I don't like days that the winds are blowing in certain directions either (like north or south around here) and when they are over 15-20 MPH. I'd rather deal with an on an off shower than wind and big waves.
I don't much take to much heed in the weather reports. As another poster said they are much the same the whole day even though the weather patterns can change quickly. Secret: weather reports always try to scare you enough to stay glued to their program . . . Why? so you can view their advertising of course. Ever notice when a storm has just hit (like a snowstorm) they always say there is always another one right behind it??? stay tuned!
What I do is look at the local radar west of me to see what I might expect to blowing over me in a few hours.
I find most of this activity tends to pass north of me where I'm located so I usually go anyway but stay close to a port for a quick safe haven where we can tie up and get off the water.
I also added a fixed VHF radio with an 8 foot antenna which will pull in Coast Guard weather breakin alerts many miles before storms reach me just because weather conditions can change in a heartbeat around here.
I'm able to pull up the National Weather radar on my Treo 755 PDA. Just yesterday, they predicted 82 degrees and clear before we left home an hour South of Lake George. As soon as I got on the water the skies darkened and I checked the radar picture and a band of red and yellows cells were moving through the area so we just pulled up to a restaurant, had lunch and avoided the down pour over the next hour. Sun came back and we went cruising only to see another band come though so I put up the enclosure and we were able to salvage the day by cruising for another few hours in periods of on and off light rain.
So I tend to only stay home if lightening, high winds or bands of cell activity that are so widespread that there would be no where to escape.
One last point: we left the restaurant when the cell activity past the region we were in yesterday. Oddly enough more bands seemed to develop on the radar map out of no where and that's why we had more rain hit us.
So none of this is a perfect science, but don't screw around with lightening.
My Wife and I watched several boats go by us in the driving rain yesterday "pulling kids on tubes" and it was thundering all around us at the time. Luckily, there was no lightening yesterday.
When it comes to lightening and knowing when to be on or off the water . . . Be really aware of what MORON parents you might let your kids go boating with!
There are plenty of morons around, I see them every time I go boating. Don't let anyone in your family become a statistic because you thought the captain knew what he was doing. 90% of them don't IMO.
Always check the radar at Weather.gov for your areas local radar for the type of rain before you go out. Here is a snapshot of what I saw this AM and why I chose to stay home today. It's because of the widespread cell activity in the ones in red, yellow and orange colors that kept me home.
Like I said before, even though there are no patterns to the west of me right now. What can happen because we have very humid hot conditions today cells can form instantly right over us because of the hot humid air rising and hitting the cooler air higher up. You won't see that happening quick enough to get out of harms way on mountain lakes!
I can pull up this same picture on my Treo, but by that time I'm already on the water so check it BEFORE you leave: