You have to carry them at all times, but do not have to wear them but must be "readily accessible". There's also a "cold water" requirement, they must be worn at all times from November 1 to April 30 Summary
Best to get the type That keeps your head above water if you get knocked out... You have to respect water.. within a blink of an eye it's over... There are regulations that in the cold months you have to wear them on Fish Commission lakes. It's the law.. Remember you can't put your life jacket on after your knocked out in the water.. Semper Paratus!!!http://www.uscg.mil/hq/cg5/cg5214/pfdselection.asp#recreational
If there is a chance your going to end up in he drink wear a vest.
They are not easy to put on in the water when one floats by or somebody tosses one to you.
If you want to have the freedom to move about but still want the security of a life-jacket get inflatable ones. They will keep you afloat if you go below the water surface and inflate automatically before you find yourself in to much trouble.
They will roll you onto your back but do not support your head if you are unconscious.
I think the time is very near till everyone out on the water sailing, under power or drifting will be required to wear a PFD.