www.iFourWinns.com

good info there, ive read that.. so just be aware of riser height as a preventive measure and also watch excessive idling and run at a higher rpm before shutting down to blow water out??

What has happened is that the valve timing has been made more aggressive on later model engines (not sure when they changed exactly) and this exacerbates exhaust reversion (pulling water back up the exhaust, under deceleration). Note that this can happen even if the engine does not diesel, or backfire. It can happen as part of normal operation. This article describes it well:

http://www.michel-christen.com/ExhaustElbows.pdf

Water Ingestion
Engine cooling water mixes with the exhaust at the end of the exhaust elbow to go overboard.
Under certain conditions, a fine mist or droplets of water can be drawn backward into
the exhaust passage of the exhaust elbow while the engine is idling. They can travel backward
until they ‘wet’ the vertical exhaust passage. When the engine is shut off, these droplets
flow downward and collect in the exhaust manifold runners that go to the cylinder head.
In saltwater areas, the water evaporates and leaves a salt crystal deposit in the runner. Over
time, these salt crystals will cause rust to form on the exposed surface of an open exhaust
valve. When this valve sticks, it will cause more water from the exhaust elbow discharge to
be drawn backward into the engine.
This condition is more likely to occur on engines that have exhaust systems that exit out the
boat’s hull. Sterndrive engines that have thru-prop exhaust are least likely to see this condition.
This condition is seen more in saltwater areas, but rarely seen in fresh water areas unless
there is a poor engine running condition. Look inside the exhaust passage (at hose end
of the exhaust elbow) to see if a salt or rust trail is present from that point on backward toward
the manifold. If there is, the engine could have water ingestion.

1. All engines have valve overlap, even 4 cylinder engines. Gen+ V6 and V8 engines have
more valve overlap than the older engines. The higher horsepower big block V8 engines
have more than the base big block engines. This ‘valve overlap’ coupled with the ‘tuning’
of the boat’s exhaust system can cause water ingestion. Valve overlap is more likely to
cause water ingestion at idle or low engine rpm than it is at higher engine rpm.

2. Extended idling before shutting the engine off.
a. This affects an engine with thru-hull exhaust outlets more that thru-prop exhaust
models. Because of ‘No Wake’ zones and other conditions, engines that idle 30+
minutes prior to being shut off, tend to be more likely to have water ingestion. Quiz
the boat operator about their idle times. One way to stop or minimize this condition
is to increase engine rpm (in neutral gear) to 1300 for 45 seconds then slowly return
throttle to idle position and shut the engine off.

3. Throttle ‘chops’.
a. Doing ‘throttle chops’ can cause water exiting from the exhaust elbow to be drawn
backward into the engine on either thru-hull or thru-prop exhaust engines. Also, it
can happen with the boat in the water or on a trailer with a flush device. Caution technicians
and boat owners about doing ‘throttle chops’.

4. Missing internal shutters in exhaust tips (sterndrive engines only).
a. Sterndrive engines with thru-transom exhaust are more likely to have water ingestion
if the internal shutters are missing. The exhaust hose length is short on these
installations. The internal shutters help break up the ‘tuning’ effect of the short
hoses. Install exhaust tips that have internal shutters that will not burn out or break
off. Installing exhaust tip silencers is another option. If there is room in the exhaust
hose, installing the Exhaust Resonator Kit will help correct this condition also.

5. Silent Choice type exhaust systems (sterndrive engines only).
a. Sterndrive engines equipped with Silent Choice type of exhaust system normally will
not have water ingestion if the boat owner directs the exhaust thru-prop while in ‘No
Wake’ zones or while idling for an extended time. If the exhaust system is in the ‘thruhull’
mode during this low speed operation, the engine may ingest water if the internal
shutters of the exhaust tips are missing.

6. Boat exhaust systems (engines with thru-hull exhaust only).
a. Engines with thru-hull exhaust systems can have a ‘tuning’ effect. Exhaust hose
length, 45
, or 90
fittings, location of collector and/or muffler and the thru-hull exhaust
tip all can affect water ingestion. The only way to test for this condition is to
monitor the amount of water that collects in the runners of the exhaust manifold after
idling for 45 minutes. Engine has to be at normal operating temperature before starting
the idle test. Using Exhaust Resonator Kits in the exhaust hoses will help stop
the ‘tuning’ affect in the exhaust system. The Mercury Parts Exhaust Resonator Kit
does not cause any horsepower loss.

The availability of different styles of risers/elbows I can't say but I know that Volvo did use a type of vaccum break set up that had hoses hooked up to the exhaust risers. The idea of this was to keep vaccum from forming in the exhaust which can suck water back in under certain circumstances.
Making sure your risers are high enough (checking static water level with the boat in the water) and making sure you use the updated Volvo riser to manifold gaskets (there was an updated part for these to prevent leaks), and making sure the vaccum break hoses are hooked up right and not clogged at the riser ports will help a lot.