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#1 is the cylinder that goes on the 2.5L motors . . . usually b/c of a stuck thermostat, worn impeller, or bad poppet valve.
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#1 is the cylinder that went on my '91 150 XR4. Ate a ring six months after I bought it. Had low compression only in #1 cylinder when I bought it but I took a chance 'cause the price on the boat was so good. Had the powerhead rebuilt and no problems in three years. I change the impeller every two years. Some do it every year, but my water pressure gauge still shows very good pressure after one year.
A note on the poppet valve; if it's installed incorrectly, as mine was during the powerhead rebuild, it can result in excess cooling water pressure and air in the cooling system. The poppet is basically a pressure regulator. If you look at the cooling system diagram posted by bigshrimpin, you'll see that the hose to the poppet is attached at the high point of the water jacket. When your impeller is nice and fresh, it produces about 25-30psi of pressure at speeds above idle. Because the water pickup on the lower unit sometimes picks up air, there has to be a mechanism to purge the air from the cooling system. The poppet purges the air and bleeds off excess pressure at the same time.
As it was explained to me, if the poppet diaphram is installed incorrectly, it don't bleed off pressure or air. The result is high cooling water pressure and the engine still gets hot because there's air pockets in the cooling system. Mine's fixed now, but it was a learning experience.
Disclaimer: I'm no mechanic. There are plenty of folks on this site who know more than me about this stuff. But I like to tinker and learn about how things work. If anyone sees something I've got wrong, please feel free to step in and make corrections.
Dave