Paul A is partly right, the pan head - the "factory head" - is formed during the manufacture of the rivet . But when they driven hot, as these were, the shop head on the other end is formed by the snap, which is the tool inserted in either the rivet gun or horseshoe rivetter.

Look at the shop heads on the rivets closest to the floor and you can see some where the rivet was slightly short, and the edge of the snap has gouged the plate as the head was formed. And there are a few where the rivet was slightly long, and there is a "rag", or flash around the head where the excess metal has been squeezed out around the snap.

we used Steinke hoods. Lock your thumbs together with hands extended overhead to break through any debris that may be on the surface. Look up & holler "ho, ho, ho" all the way up; this, to make sure that you exhaled air as it expanded in your lungs due to decreasing water pressure as you ascend.

I'm pretty sure the fellow on the left will not be looking up. If he does, he won't be saying "ho, ho, ho".

That tank behind them has a port hole window in it. I'll bet that's the 100 foot training tank. As for the rivets, they're made with the cone shaped head and the other end is peened over with an air hammer after inserted so it ends up domed shape.

As usual in a Shorpy photo I look at the tiniest of background details. The cone head rivets and flat head ones. Why the mix on the same boiler, tank, or whatever that is?

choice of footwear for swimming!

How could we make this image an Internet meme?

Rute Boye and tcrosse nailed it.

Read all about it here: http://en.wikipedia.org/wiki/Momsen_lung

The somewhat pained expression on the faces of the two gentlemen on the left and right might have something to do with the location of the straps on the lower part of their bodies.

It looks like various iterations of the Momsen Lung under test at the US Navy Submarine School escape training facility at New London, CT.

Sub school submarine escape training.