Water temps in the 50s and air in the 70's I'd probably use a 3mm full suit with booties, hood, and spray top as necessary for comfort.
SRM has great information. The only thing that I'd add is that for me (and it really is a personal thing) once the air temperature is into the 70's, I find that wearing a wetsuit can be very uncomfortable because all I do is sweat, which in turn can cause you to dehydrate quite quickly.
When the water is still cold, and the air temperature is in the 70's, I'll bring the full wetsuit along, but by then I'm usually wearing my wetsuit shorts, and a separate vest.
My usual system looks like this,
Rashguard - or moisture wicking long sleeve shirt
Spandex underwear - or even bicycle shorts (moisture wicking, fast drying, not cotton)
Dry pants (with built in feet)
Socks under the dry pants on feet
Neoprene booties over the dry pants on feet
These are listed from Body, to outer layer. I realize that you don't want to know about my underwear
but I'm sailing on a Bravo, so if things get really crazy, I can furl my sail in an instant and, if necessary, change completely into my full wetsuit. You need a layer under the wetsuit shorts to do that without scaring the crap out of the passing boaters
The other thing is that I'm able to take on and off my spray top without removing my life jacket. That helps for when you get hot, you can dip your hands or arms in the water to help keep you cooler.
I use a wetsuit a lot more in the Fall, when air and water temperatures are closer, but in the Spring, you can have very hot days on
the water, while having very cold temperatures in
the water. It's a bit tougher to dress for Spring than for Fall. Dress for safety first, and comfort second, but with the right selection of clothes, you can find a solution that meets both of those requirements.