Probably the impeller. They only last a year or two before the rubber starts to break down and don't move the amount of water they should. I've never worked on a Honda but with OMC and Mercury/Mariner smaller outboards you can get to the impeller (which is driven off the shaft) by dropping the lower unit with 3 or 4 bolts. Check to make sure there aren't any blockages in the water tube that draws the water up from the impeller.

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1980 300TD-China Blue/Blue MBTex-2nd Owner, 107K (Alt Blau) OBK #15
'06 Chevy Tahoe Z71 (for the wife & 4 kids, current mule) '03 Honda Odyssey (son #1's ride, reluctantly) '99 GMC Suburban (255K+ miles, semi-retired mule) 21' SeaRay Seville (summer escape pod)