TOM, I have no experience with Yanmar
engines. However, I have had a leak on the seawater pump on my Nanni engine. The leak was between the shaft coming from the engine and the gasket
There was no need to remove the whole pump: the gasket
was held by a circular spring (known as a "Circlip" here), accessible from the inside of the pump. I just had to buy the special pliers to remove the spring.