Quick update for those (and thanks to all for feedback so far!) who are
following this tread. Spent Sunday afternoon at the marina still
trying to diagnose the source of the overheating problem. First I
disconnected the hose coming from the transom/leg and going to the
thermostat housing: I started the boat and saw a really good flow of
water coming out of the hose. To me, this would indicate that the raw
pickup of water on the stern drive is not plugged and the two-month old
impeller appears to be pushing water through the hose as it should.

Then I pretty much disconnected, sequencially, all cooling related
hoses going in to and coming out from the engine block. A flow of
water could be seen although I do not know if this flow was normal or
lower than what it should be. The only place that I did not check is
where the hose re-enters the thermostat housing and presumably find its
way to the manifold/riser and then out. So if I am still thinking
blockage, this may be one more place to look: is it easy to remove the
riser cover? With blockage or clogging being a less likely cause, I
am once again looking for other culprits.

Again, restating that I am not very mechanically inclined, I am
starting to think that my problem may be related to a defective water
pump (not the stern drive (impeller) one but the "engine-circulating"
water pump (the one that is spun by the alternator belt I believe). I
see no physical evidence of a defective or clogged water pump but if
that water pump was not turning properly (sliding or whatever) or if it
was clogged or defective (but not 100% broken), could it explain why
when I get on my way and increase RPM, the problem seems to eventually
fixed itself (cooling process starts, needle drops to 150F and all is
well). Perhaps, there is a point where the water pump turns "enough"
to enable the cooling process to happen? Could this explain why the
overheating appears to happen primarily at idle or low speed?

When the boat runs at idle for the first 5 minutes (until the needle
creeps to 200F+) the boat sounds good. No weird or clinging noise or
whatever. As far as I can tell, oil in the engine block is clean
(although I admit I am not sure I would be able to see the difference
if there was water in it - unless it is very obvious). So I am
thinking/praying that I may not have a major problem yet and that the
problem may still reside within the (raw) cooling system itself.

Any comment, advice, suggestion regarding probability of a clogged
manifold/riser or the possibility of having a defective
engine-circulating water pump (even though no external sign of it not
working properly)? Sorry for the long post! Andre

PS: yes, I recognize (and regret!) my lack of mechanical knowledge and
yes, I am looking for the service of a boat mechanic to help me out.
However, they unfortunately all seem pretty busy at the moment!