Boat motor problems

by Ethan
(Missouri )

Ok so I bought a 77 hurricane 15 foot bass boat with a 77 Johnson 70 HP 3cyl motor on it well the motor runs great when u run the hose through it and run it out of water but you put it in water and at full throttle on the shift lever it only goes one or two miles per hour it doesn't go to full speed but it does seem like it will when its out of water we checked and made sure no air was getting in the fuel lines and we checked the pressure the engine is still good we also checked the spark pluggs and one seems to not be sparking good on the 3rd carb but it when it does spark it puts out a white bluish spark we checked the shearing key its good we don't know what else it could be cause it performs like it will run 40 or 50 miles per hour out of water but once you put it in it doesn't go could it be the lower unit or what I'm stumped! Pls help

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