I'd be more comfortable with a temperature probe in the exhaust/water mix immediately downstream of the mixing
elbow than I would one strapped onto the outside of the hose.
The problem with pressure sensing devices is that the pressure regime within the
exhaust is very active. Rather than any steady pressure there are large surges caused by "slugging" in the
exhaust gas/water stream and when motoring in a following sea the water level quiet often submerges the exhaust outlet.
Using a differential pressure
sensor with the low side connected to the output of the
raw water pump and the high side connected into the exhaust manifold just upstream of the mixing
elbow, in order to maximize the differential pressure drop without installing any further restriction in the flow, with the voltage output passed to a voltage comparator may be more effective than just a switch.
These are available at RS Components in various pressure ranges.
This arrangement would also allow monitoring of the flow rate if a calibrated meter was included.