How effective are life jackets at keeping people afloat? If you are
conscious, is it possible to drown even if you are wearing a life
jacket?

I've googled, and found the same definitions of type I, II, III and so
on. While this was helpful, it wasn't super clear regarding proper
size, buoyancy, brands, etc..

Which type is the most effective? Which brands are the best, and are
there a few online knowledgeable resellers that provide real help in
selecting one? What should the ballpark cost be?

I'm 6'3'', 240lbs, and wear a size 44 jacket.

The use would be occasional water skiing, recreational boating, etc.
While comfort and style would be nice side benefits, I'm mainly
interested in it saving my ass, whether it's bulky or costly really
doesn't matter a whole lot.

Thanks.

Keith