If they're nice bibs, I'll host them on the rec.scuba photo gallery
page if you send me a copy.  They can even be clothed :-)

A bit more seriously,

A human body is pretty close to neutural buoyancy; a person's lung
volume is usually 'just enough' to push us over/under this buoyancy
balance point in freshwater.

Saltwater, being more dense, tends to create a slight positive buoyacy
bais.  So too do body compositions with (euphamistically) 'higher fat
content', since fat is less dense than muscle.

What to do about it?

One option is to alter your body's net effective density.  Exhaling is
the quickest way, but can have some complications and limitations.  If
nothing else, you have a smaller reserve of O2 in your lungs for your
blood to exchange with, so the time you can stay underwater is
reduced.

The second option is to wear some ballast.  Essentially the same thing
you do to counteract additional buoyancy from adding a wetsuit.

However, for both of the above, something to keep in mind is that the
air remaining in our lungs will be compressed by ambient pressures as
we go deeper, so as we freedive, our bodies lose some of its natural
buoyancy.  As such, even with a full breath of air, we may not
necessarily still be positively buoyant when we are, say, 20fsw or
deeper.  As such, adding extra ballast has to be done  cautiously.

The third - and IMO best for snorkeling - option is to "use what you
have" better.  In general, what I'm referring to here are techniques
such as surface dives.

The basic physics behind a surface dive is that you're lifting part of
your body (your feet) out of the water, where since they are now
sticking out in the air, they're not displacing water, so this will
act as a force to push the rest of your body further down into the
water.

Performed reasonably quickly, and you'll overshoot your neutral point,
which is a good thing because what you do with a surface dive is to
then kick your feet to drive you under.  Fortunately, the jacknifing
for a surface dive gets you pointed straight down with your fins
behind you, so you're now pointed in the direction you need to go with
your fins in the orientation to be the most efficient.

For a few body-types, just getting down to the bottom of the pool is
enough of a pressure change to compress the air in their lungs and
they can hang out at the bottom, looking for quarters in the drain.
Most of us will start to float back up as soon as we stop kicking,
although at varying degrees.  Sometimes, a short exhalation 'burp' can
be useful then to trade-off some O2 supply because it can reduce
exertion level (the demand for metabolizing the same O2).

Just be careful when experimenting.  You don't really need to
hyperventillate for any of this.

-hh

hey, no problem.  one advantage, when we had to tread water/float for the
dive pre-lims, I could go on forever

 I can float with a six-pack on my chest, and keep my face above water.

 Fact.