I like the servo collapsing the BEC of the receiver........... Disconnect THAT steering servo. Plug in another servo. Do left & right turns with just the new servo connected. ..... Did that servo allow full power during a left turn on the joy stick ?
Plug in the boat servo..........DISCONNECT the servo rod from the rudder. Do a full power left & right run. O K ? yes ? The bind is 1 of the ends of the adjustable threaded clevises......
Check if the threaded rod of the clevis is STICKING INTO the rudder arm only during a left turn. I did that on 2 models.