This is most likely a carb problem. When the motor is on land with no load it doesn't require much fuel to rev up but in the water the exhaust back pressure and more importantly the load of trying to push the boat requires more power (Fuel). It could also be a timing issue but fuel is more likely. How old is the motor? Do you have fresh gas? Has the motor set for a long time with untreated fuel in the carbs. If so you may need to pull the carbs and clean them. Water in the fuel is also a common problem especially if you are useing gas with ethanol in it. The primer bulb in the fuel line could have bad check valves, If you have a built in fuel tank in the boat there should be a check valve in the fitting where the fuel hose connects to the gas tank that prevents gas from siphoning out of the tank in case you get a hole in your rubber fuel line so the boat doesn't fill up with gas or gas fumes and cause a fire or explosion. These check valve cause a lot of problems on outboard motors especially older ones or if the fuel pump is a little weak the pump isn't strong enough to pull the check valve open. If you pull that hose off and unscrew the fitting from the top of the gas tank thye check valve is simply a spring and ball bearing inside that fitting. Make sure you are in well ventilated area and have your bilge blower on before messing with the fuel system!
This answer was edited by Monogyny644 93 days ago.
Reason: additions
