Seat the impeller by twisting the driveshaft clockwise while seating the impeller housing. If you get it backwards, the vanes MAY self correct, or may not, and then shortly thereafter fatigue and break, sending little pieces of rubber to clog your water jacket and over heat your motor costing you lots and lots of money and frustration.
If you're not sure if you seated it correctly, by all means, take it apart and do it again.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury