Ok in which case 85 to 90C is too hot and 74C may well be the correct thermostat. The workshop manual I linked to does not cater for seawater cooled engines or even marinised engines for that matter. Sea water cooled engines have to be run at lower temperatures than fresh water cooled ones otherwise corrosion and scaling rates would be excessive.

I cant help with the fitting on the manifold and I cant think why there would be a pressure relief valve on a system which is not pressurised. Can you post a photo then perhaps someone will recognise it.