Let's walk things through here, as there seems to be some confusion regarding
wet exhaust system installation, confusion caused by people looking at line
drawings from advertising brochures.

Let's take a hull with a seacock below the waterline. Just the seacock. Open
it and water flows into the hull, right? Close it and water stops. Why? The
outlet is below the waterline. Everybody agrees on that, right.

Now, let's put a length of hose on the seacock. (We called it a "hose", but
that "hose" could be the entire engine raw water cooling system. Still, we
will call it "hose".) Now, open the seacock and what happens. If the hose
outlet is below the waterline, water flows into the hull. Of course, everybody
says, the outlet is below the water line. Now, make the hose stop right at the
waterline, water stops at the top but doesn't flow into the hull (unless the
boat rocks or heels). Now, make the hose (remember, "hose" can mean the entire
engine raw water cooling system) **at least** six inches above the water line.
No water flows into the hull (of course) while the boat is level and there is
little chance water will flow into the hull under most happenstances of rocking
or heeling. Great.

Now, let's take that "hose" that opens **at least** six inches above the water
line and turn on its pump (i.e. start the engine and running its raw water
pump). What happens? Of course the engine pumps water out the top of the hose
and water flows into the hull. This water must be removed from the hull (or in
this case, the wet exhaust system), and is removed by the pressure of the
burned exhaust gases from the engine. Hence, a "wet exhaust". Notice that
when the engine stops running the water stops flowing because the outlet of the
hose is above the waterline.

"But wait a minute," somebody says, "what happens if we place a u-shaped end on
the hose and put an anti-siphon valve in it?".

Same, same **as along** the outlet of the hose is above the water line. The
engine stops the water flow stops because the outlet is higher than the water
line, the anti-siphon adds nothing but expense.

This time lets make the end of the u-shape below the water line (dumb, dumb,
dumb but let's address that in a minute). *IF* the top of the u-shape is below
the waterline the whole thing acts as if the hose were straight up. In other
words the entire hose is below the water line and if the seacock is open water
flows into the hull and/or exhaust system. Note that any anti-siphon valve
installed anywhere in this system is useless, for there is no siphon just a
hose that starts below the water line, always stays below the water line and
ends below the water line. Open the seacock water flows in and MUST be pumped
out by the running engine. This is a forbidden installation. Dumb, dumb,
dumb.

Now, let's make the **top** of the u-shape above the waterline but the outlet
below. What happens when you open the seacock? Not much, the water flows into
the hose and up until it equalizes and then stops flowing. But, turn on the
engine to a pump water and fill the entire hose with water -- from the seacock
to the top of the u-shape and down to the outlet which is below the water line
and open to the inside of the hull and/or exhaust system. Water flows and must
be pumped out (by engine exhaust pressure).

Here comes the potential for trouble.

First, when the pump stops there is in fact a siphoning effect (because while
the top of the u-shape is above the water line, the outlet is below). Under
these conditions (should the exhaust installer fail in his job, AND the boat
designer put the engine very low in the boat) there is a need to "break siphon"
by installing an anti-siphon valve. Note that this **requires** the
anti-siphon valve to be above the waterline under all potential boat heeling
positions.

Second, anti-siphon valves fail due to crud in the water. They always do
eventually. The solution for this is regular preventive maintenance. (There
is another solution to avoid anti-siphon crudding up but I have never seen it
but once.)

In other words, an engine with its water injection point of its wet exhaust
below the water line is in danger of at some point filling the exhaust system
with salt water with the engine shut off. A competent surveyor would fail a
boat with such an installation. If there was no way in hell the water
injection point could be moved higher (due to stupidity or deliberate choice by
the boat designer) then the insurance company could decline coverage or
increase its premiums to cover the greater potential for boat loss. Sooner or
later the wet exhaust is going to flood unless the boat owner is deligent about
keeping the anti-siphon clean AND is damned lucky nothing flukey happens
between maintenance events.

The solution is to mount the water injection outlet **at least** six inches
above any potential water line.

It is in the installation instruction that come with a water injection elbow.
RTFM, guys. RTFM