The more critical temp is the outlet temp of the RAW side .... or rather the difference (∆T) between the inlet and outlet of the RAW side.

FIRST check. You need to be SURE of the volumetric output (gallons per minute at spec. rpm) in the RAW water side. A 28 hp engine's correctly sized Hx will typically need 3 gallons per minute at 2000 rpm output. Heat transfer is dependent on proper mass transfer of water to gain the correct heat transfer. Run the engine at 2000 rpm, stand in a dinghy at the outboard water exit with a bucket of known volume and with a stopwatch, etc. measure the output vs. time. If less than 3 gpm/2000 rpm you have a blockage restriction. Of course you need to look at the Volvo manual exact specs. for this rating of mass transfer.

Second check. (after clearing all the blockages/resistances from above). Get an infrared gun-type thermometer. Measure/ascertain the DIFFERENCE in temp. between the inlet and outlet of the RAW water side of the Hx. If the difference is GREATER than ~15deg.F (known as an 'approach' temperature), the Hx is fouled and needs to be 'descaled' .... (note the input raw water should be less than ~85 deg. F to make this judgement). Use a commercial boiler descaling compound such a RydLyme, etc. (do NOT use muriatic, etc. acid) to descale the engine AND the water passages of the exhaust manifold. Commercial boiler descalers are pumped into the 'loop' and are biodegradable .... cant be dumped overboard when done. IF scaling of the raw water side of the Hx is a constant problem, consider to get a thermostat (fresh water side) that opens at a lower temp. instead of the 'normal' 180 ..... say 160 or even 140 if the scaling is severe.

If the above doesnt gain a better heat transfer, then dismantle/inspect the water injection section of the wet muffler for soot, 'coke', etc. blockage in the 'exhaust gas' section of the 'injection elbow' AND the 'gas side' of the exhaust manifold. If you do find a lot of 'coke' in the injection elbow and exh. manifold ..... CLEAN all the 'fungus' and 'alkene deposits' out the fuel tank and STOP topping off your fuel tank. Only fill the tank the minimum NEEDED plus some reserve .... and you wont have so much 'degraded' fuel in the tank that doesnt burn well in the combustion chamber and easily forms 'coke' on the walls of the exhaust system.

:-)