OK, let's look at the fallacies of both your scenarios:

Rick wrote:
Lets look at the safety of both suggestions.

1) Nitrogen liquid will boil to gas. The gas is an asphyxiate. Many people
have died from nitrogen. Air contains 21.5% O2 mostly the rest is N2. If
you breath pure N2 the first breath you pass out and the second breath brain
damage and the third death.

GROSS exaggeration. This assumes you purge *all* the lung volume with
each breath (when it's typically only about 30% or less) and that there
are no O2 reserves in circulating hemoglobin or stored myoglobin.
Neither is the case. It also assumes (at 12-15 breaths per minute) that
brain damage occurs in 10-15 seconds. Right.

Think about passing out when reaching for
something in the ice chest. If you fall in your dead; if you fall out you
will probably be ok. The next day you will be ripping out your N2 system.
A closed boat can accumulate N2 which will displace air.

N2 evaporation does not *displace* air, it dilutes the O2 concentration.
For a sleeping individual, oxygen concentration does not become
dangerous until it reaches about 10%. That means you'd need to
evaporate sufficient N2 to equal the volume inside the boat, and you'd
have to do it without ventilation. Not very likely. My boat doesn't
hold 30 PSIA, does yours?

Maybe get you in
your sleep or when you go down for a cold one.

2) CO2 is heaver then air and would accumulate in the low parts of the boat.
Same issue as with N2 but it would at least give you some warning signs.

It gives exactly the same warning signs as N2. No more, no less. You'd
require the same volume of subliming CO2 as evaporating N2 to cause O2
deprivation issues.

Bottom line, you must have ventilation in any sleeping environment. You
exhale CO2, you'll recall, so you'll die in a sealed box or without
subliming CO2 or evaporating N2. That's just common sense.

Whats wrong with a little water from melting ice. You are on a boat
right???

I don't use CO2 in my boat, but I've been using it for years in my
Vanagons' Dometic NH3 "refrigerator", and it's posed no problems. I put
in about 2 lbs which lasts about 3 days. 2 lbs sublimes to about 510
liters, in a van with an interior volume of approx. 6800L. When you do
the math, that results in a total O2 dilution of 7.5%, or a final O2
concentration of 19.9% which is well above OSHA's TEEL limit of 19.5%
for a confined space entry. This again assumes that no ventilation
occurred over the 3 days.

When you do the math, it just isn't a problem when using any reasonable
amounts, and with only normal ventilation (which should *always* be
ensured under *any* use conditions).

Keith Hughes