The circled part in the photo is the aqualock and is unlikely to be the cause of the water jet from what I assume is the siphon break hose from the vent loop. Usually the water flow from an overboard
siphon break is a steady flow at full power and a slight trickle at idle, rarely a jet of water. The observant "passing yachtguy" might be right about a raw water
system restriction causing the jet but its more likely to be in the mixer elbow
seen under your red arrowhead. Incidentally the raw water
circuit seems to be higher on the 4LH series, possibly due to the longer trip through the engine
coolers rather than just the short route
from the pump
, into the heat exchanger
and then the exhaust via the mixer elbow
. One of the boats here has twin 4LH's and it's blown out a raw water hose on 3 occasions (which is unusual) so maybe the high vent loop pressure is normal and not a symptom of high back pressure.
In your photo it's difficult to see where the vent loop piping might be.