SeaTalk is a 9bits serial communication protocol. Around 2012, Nick Gammon developed a HardwareSerial library, it was limited to IDE 1.0.1, and required the replacement of native IDE library. see here: Topic: "Mods to HardwareSerial to handle 9-bit data"
Was, since the original issue, any HardwareSerial library update?
Is it possible to move the Hardware serial library to library folder, leaving IDE package untouched?
link to the library: http://gammon.com.au/Arduino/HardwareSerial9bit.zip