Try this Spearfishing / Free Dive training programme I'm on - you won't believe the results!

Dive Training : Advanced programme for Free Divers and Spearfishing

Building upper body strength –
(great for increasing lung capacity, swim & speargun load strength)

Just came across this exercise suggested for all spearfishermen to build
muscle strength in the arms and shoulders. It seems so easy so I
thought that I'd pass it on to my dive buddies and even my family. The
article suggested doing it three days a week.

1. Begin by standing on a comfortable surface, where you have plenty
of room at each side. With a 2kg potato sack in each hand, extend your
arms straight out from your sides and hold them there as long as you
can. Try to reach a full minute, then relax.

2. Each day, you'll find that you can hold this position for just a
bit longer. After a couple of weeks, move up to 5kg potato sacks. Then
25kg potato sacks and then eventually try to get to where you can lift
a 50kg potato sack in each hand and hold your arms straight for more
than a full minute (I'm at this level).

3. After you feel confident at that level, put a potato in each of the
sacks.