Opinions vary, but for most of us, the answer is a wooden band gun.  It's
powerful, accurate, quiet, well balanced and simple to maintain and repair.
Overall, they're negatively buoyant, but just barely.  When loaded, they sit
with the point on the bottom and the butt up where it can easily be seen.
When fired, the spear sits on the bottom and the gun floats . . . assuming
you don't have a spare shaft attached.

There are four brands I know.  JBL makes a series called, so help me, Woody.
I hate the name for obvious reasons, but the guns seem to be pretty good.
The JBO is probably the least expensive of the group.  As near as I can
tell, the Sea Hornet and the A.B. Biller guns are the same.  This is what I
have.  They are more expensive than the JBLs.  They may be better.  They are
quicker without a line attached, something suitable for reef fishing, but
not blue water fishing.  This, in my group, is referred to as freeshafting.
The top of the group I'm most familiar with is the Riffe.  They're the
fastest and quietest, but also the most expensive.  Because they use the
line to hold the tip of the spear in place, they can not be freeshafted.