Re: 1988 Mariner 40 hp - does it have a thermostat?
Thermostat is located under the cover directly above the top sparkplug (cover kinda looks like a figure 8 on it's side). There is also a poppet valve under that cover (a kinda poppet - not like on modern Merc's - looks more like the thermostat in your car - called a pressure relief valve back in the late 80's).
On this particular model the tell-tale is getting it's feed from the exhaust plate (where the water first enters the powerhead), so it's "normal" for it to start flowing as soon as water makes it to the powerhead (the water channel in the exhaust plate feeds the channels in the powerhead itself).
On the downside of this particular design, while it tells you the water pump is working fine it doesn't tell you if the water is getting past the thermostat or circulating properly through the powerhead.
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)