In the Marine Corps boot camp of the past you had to pass water survival training by jumping in with full gear (cammo's, boots, a dummy rifle and Alice pack) - yep, it sucked - even tho the pack floated minimally.
My floatation point dead still in the water is roughly equivalent with my hairline (i.e., I do not float). Both fresh and saltwater, I just plain sink.
Wear, and I'll say this again, wear - not just buy, but again in case you missed it - wear a PFD. At the minium a manual inflatable fanny pack style PFD.
It causes minimal discomfort, doesn't affect sun tan lines - you can adjust it fore and aft for those of you that don't want lines at all.
If you can't swim or tread water as you are normally dressed for at least 20 minutes or you are sailing alone you are literally taking your life in your hands every time you go out.