Before disassembling the water pump housing, rotate CW the drive shaft with hand and check if there's a rotating restriction going on, if so the internal impeller body holds well onto the drive shaft's groove.
Before assembling back the lower leg to middle motor, place it inside a barrel with sufficient amount of water and with a drill machine rotate the shaft CW at speed, if pumps lots of water out the grommet, the issue could be located in the upper grommet that connects the water tube to the exhaust tube's base, probably not sealing well due to being hardened, cracked or with tube failure undergoing excessive water loss. To rule that out will need to remove the entire powerhead from the middle pan to visually inspect that out.
Other issue to be aware of is : if the motor was used its entire boating life in salt water could already have all its water pasages with excessive salt layers, crusts formations which will lessen the peed out water through the water indicator ? Nope, flushing the motor all day long with fresh water after use won't save it from collecting salt layers in the short term. Assume that have not tested the motor that long at speed to check if with overheating issues, right ?
Happy Boating
