Hi Duncan-m
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.