Filling with water would be a bad idea I'm thinking.
When you introduce air into a container under the waters surface, you increase the pressure in that container. The farther below the waters surface you go, the more pressure on the air in the container. This means the pressure in the container is is significantly greater than the pressure on the outside of the container. This is why we use balloons to float sunken ships to the surface and why rafts float.
The following is an excerpt from a page I found on the internet. It's a discussion about a test where you crumple up a piece of paper, put it in an empty glass then turn the glass upside down and press it into a bowl filled with water. The paper is compressed. The question is why. There are two theories that govern this, Pascals Law, and Boyles Law. Feel free to read up on them. Anyway, this guy provides a fantastic answer. This will help to explain why I was putting so much thought ino how to keep things underwater.
"The thing you want to keep in mind while thinking about this experiment is
that the air in the glass is under pressure. Let's start with just a glass
right-side-up and full of air. The air stays in the glass because the air
above it is pushing down. (If you took the glass into outer space, the air
would whoosh out immediately, spread out far and wide, and eventually--
attracted by Earth's gravity--fall until it landed on Earth's atmosphere.)
One way to figure the pressure of the air in the glass is to imagine a
column of air one inch square extending from the glass straight up as
far as the atmosphere goes (a few miles). This column of air weighs
around 14 pounds, and the air in the glass is preventing the column
from falling, so the pressure in the glass must be around 14 pounds per
square inch.
By the way, if you had a tire full of air at this pressure, you would
say the tire was flat--i.e., that it had NO pressure in it. This is
because we take normal air pressure for granted and because tire gauges
measure the DIFFERENCE between the pressure inside the tire and the
pressure outside the tire. Don't let this throw you. If you took the
tire to outer space, it would feel like it had 14 pounds per square
inch of pressure in it. Now if you double the amount of air in the
tire without changing anything else, you will double the pressure. I
won't prove this, but it probably seems a reasonable result once you
realize that pressure really is the combined effect of all the air
molecules that collide with the tire walls. Twice as many air
molecules per unit area would pound twice as hard. You could also
increase the air pressure by reducing the volume of the tire, e.g., by
squeezing it. If you cut the volume in half, the pressure will
double. Finally, you could increase the pressure by heating the air.
This would make the air molecules zip around faster and collide harder
with the walls of the tire.
If we go one inch deep into the water, the water pressure must be
sufficient to hold up not only the column of air above the water, but
also an additional cubic inch of water, which weighs around 16 grams or
0.6 ounces. So now we know the pressure at every point in the water:
14 pounds per square inch at the surface, increasing by 0.6 ounces per
square inch for every inch we go down into the water.
Ok, back to the experiment. We start with a glass full of air, upside
down, and place the rim of the glass at the surface of the water. The
air is pushing down on the water with 14 pounds per square inch of
pressure, and the water is pushing back. The surface of the water will
not go down unless the pressure on it increases. As we lower the glass
so its rim is exactly one inch below the water surface, two things happen
at the same time:
1) The water surface under the glass does go down by almost an inch, to
a depth at which the pressure is slightly greater than it used to be.
The water that used to be where the glass now is has been pushed out of
the way, and this means the level of water in the bowl has risen
slightly. Let's ignore this either by using such a large bowl that the
rise is negligible, or by always measuring from the surface of the
water. We can get away with this because air pressure doesn't change
appreciably as we go up in the air by a only a few inches.
2) The air in the glass gets squeezed into a smaller volume, so that
its pressure increases by exactly the amount the water pressure has
increased. We know this increase is 0.6 ounces per square inch per
inch of water depth, so the air pressure has increased from 14 pounds
per square inch to nearly 14 pounds 0.6 ounces per square inch--by
roughly 3%. This means the volume has decreased by roughly 3%, which
means the air surface has receded into the glass by roughly 3% of the
height of the glass.
Tim Mooney"
In essence, the upward pressure on the container is significantly greater than the outward pressure on the tanks walls from the water.
