Welcome to the Clamshell club!
As an '89 model you are likely (but no guarantee) to have the revised edition cooling - they removed the thermostat & Bypass valve because of possible overheating problems, so you basically get full flow through the whole head all the time. Makes the Idle rough as a badger's @rse, but keeps the engine happy when running.
The way to tell is if you look at your exhaust side (LH if looking towards the bow of the boat) you will see a big round bit o nthe casting (about 1" dia) on the exhaust cover about halfway up. If it has no hole, you have the new cooling system so a blockage there is less likely, if it has a small hole, you have the thermostat & valve, but order up a new exhaust cover gasket before you take the lid off as both the thermostat and poppet valve can block (one of the reasons I modified mine) Also check that hole - it is to refer the poppet valve to atmospheric pressure and if blocked with salt etc can casue problems Again a quick poke will clear it, but beware you have a rubber valve under there so don't poke it too hard or you'll rupture it!
Also probably worth making sure you have the latest version of the nozzle at the outlet - the newer ones have a smaller hole so more water goes through the engine! (a worn one will have the same effect) They are V cheap so probably wiorth replacing anyway to be sure.
I have a full set of pics of it in bits on the home PC from my rebuild. I'll dig the ones out of the top of the leg - there are (from memory) 2 gaskets to get through to get a leak there!
Other thing - are you testing on muffs or in the water? There are lots of places on the leg water squirts out of if on muffs - and some of it goes up!