I am a certified scuba diver with 250 dives and Padi advanced ticket I
have used both methods. My boat is a 33 ft Newport sailboat in San
Francisco Bay. For attaching a zinc, holding your breath and dipping
under the baot in maybe 10 small dips works great. I have done it.
For longer jobs I have made up some gear: Bought an oilfree compressor
with 2 HP electric drive so no fumes from and engine and hocked up to
50 ft hose. The compressor puts out 100 psi which allows you to go
fairly deep. To the hose I attached my second stage regulator from my
scuba gear.
There is no danger of electrical shock because the compressor is
plugged ditrectly into the outlet at the marina, the only thing in the
water is the hose.
If a person has no scuba training I would not go deeper than maybe 10
to 15 ft. I think there are several things to know, one is to clear
your ears same as when you are in an airplane, but this may not be
necessary 6 ft. Second is to have a diving mask to be able to see and
breathe out through your nose to equalize the pressure. Again at 6 ft
this may not be a problem, and third is to keep breathing in and out
while you come up so that the air in your lungs can expand. So I think
to go to very shallow depth like 6 ft it is not a problem even for an
untrained person. The cost of the compressor was $100 at Sears and the
hose another $15. The advantage is now I can clean bottom and stay
under the boat and do work as long as I like. The 2 HP compressor
supplies lots of air. The second stage regulator works fine with 100
psi. I have used this to inst6all a new rudder while the boat was in
the water. This saved me a $400 haul out. Plus I now can clean the
bottom anytime I like at no additional cost.

Tom Shilson wrote in message ...
Chuck Tribolet wrote:
100 PSI isn't enough. The intermediate pressure in a typical SCUBA
system is set at 140-160 PSI.



IIRC, from ages ago, every 30 feet of water is 1 atmosphere of pressure
(15 lb. / sq. in.) If he is going down, say 6 feet, 15 lb/sq.in. should
be fine. If he tries to go deeper, he will suck air, get none, and come
up to where he can breath.

Tom
of the Swee****er Sea