OK..... first, we need more info re; your engine model number.
Launch this web site and identify your engine.
http://www.marinepartseurope.com/en/volvo-penta-schematics-MarineGasolineEngines.aspx
My first suggestion would be to argue that the related work as per the $800 charge was futile. You may want to ask that they refund all or a portion of it....... or, that they continue diagnosing and correcting the problem on their dime!
The longer that this issue goes un-attended, the greater the risk of Engine Damage!
My second suggestion would be for future service work.
a..... Be willing to pay for diagnostics up front.
b..... Hold their feet to the fire and expect that the Diagnostics will actually identify the problem.
c..... Authorize the work scope only upon a written understanding that the diagnostics are correct, and that the work scope will indeed correct the problem!
Otherwise, you may as well sign over a blank check!
While we do not have a model number....... I don't believe that your Volvo Penta 4.3L engine will be fitted with Risers/Spacers.
You should have Elbows that will host the over-temp sensors.
If Raw Water Cooled, you will have seawater transfer ports between the exhaust Manifolds and Elbows.
When this boat was new, all works as it should. So we know that the system can be made to work correctly and without any issues.
So..... something has changed as to create the now over-heating.
...... check and/or replace the seawater pump impeller....... or if need be, replace the entire seawater pump.
...... pull a vacuum test on the seawater pump's suction hose and look for a suction breach. One small breach will play hell on the system!
...... check all related hoses for interior de-lamination.
...... look for any excessively tight bends in the seawater path hoses.
...... check for rust scale blockage within the seawater passages.
...... ** check the seawater transfer ports between the Manifolds and Elbows.
...... ** if the engine is an early model, and is fitted with the 95mm exhaust Elbow, make darn sure that the 95mm soft wall exhaust hose is NOT being compressed into the elbow scalloped areas.
..... check the exhaust outlet port "back-flow-prevention flappers" for any signs of resistance.
..... in other words, check the entire seawater path from the point of entry to the point of discharge!
(** these are important ones!)
The first image below is of a typical Volvo Penta exhaust manifold and elbow showing the seawater transfer ports.
(this is the earlier AQ series)
The seawater transfer ports must be clean and clear or rust scale.
On the AQ series exhaust system, you will see the exhaust elbow scalloped areas that I mentioned.
The OEM 95mm exhaust couplers are always too short by approx 2" of length!
Because these are too short, they inadvertently become adjusted and clamped over the scalloped areas.
Over time, they eventually become squeezes into the scallops and restrict "spent" seawater flow!
The result is over-heating!
DO NOT..... repeat..... DO NOT clamp this 95mm coupler over the scalloped areas.
If your 95mm soft wall exhaust couplers are short, purchase new 95mm soft wall exhaust hose by the foot and move the band clamps further up the elbow snout and away from the scalloped area.
But again, we need to know your engine model number.